We look at the division of 351172 by 21 
Let's see how many times 21 fits into 351172.

Advancing to step 1:
When dividing 3 by 21, we get 0 with a remainder of 3.
Write down 0 as next digit of the result. 
Result so far: 0.0
Subtract 0 from 3 to get 3.
Fetch the next digit (5) from the dividend, attach it to 3 and continue: 35 / 21

On to step 2:
35 divided by 21 is 1 with a remainder of 14.
Use 1 as the next digit of our solution. 
Result so far: 1.0
If we subtract 21 from 35, we get 14.
Include the next digit (1) from the dividend after 14, then repeat: 141 / 21

Step 3:
When dividing 141 by 21, we get 6 with a remainder of 15.
The next digit of our result is 6. 
Result so far: 16.0
If we subtract 126 from 141, we get 15.
Fetch the next digit (1) from the dividend, attach it to 15 and continue: 151 / 21

Step 4:
The number 21 fits into 151 7 times, leaving a remainder of 4.
The next digit of our result is 7. 
Result so far: 167.0
Subtract 147 from 151 to get 4.
Append the next digit (7) from the dividend to 4 and continue with: 47 / 21

Step 5:
47 divided by 21 is 2 with a remainder of 5.
The number 2 becomes the next digit in our result. 
Result so far: 1672.0
Deduct 42 from 47 and we're left with 5.
Bring next digit (2) of the dividend behind the 5 and repeat the process: 52 / 21

Advancing to step 6:
21 goes into 52 2 times with a remainder of 10.
Write down 2 as next digit of the result. 
Result so far: 16722.0
If we subtract 42 from 52, we get 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

Step 7:
21 can be fit into 100 4 times, resulting in a remainder of 16.
The number 4 becomes the next digit in our result. 
Result so far: 16722.4
The remainder is 16 after subtracting 84 from 100.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Advancing to step 8:
When dividing 160 by 21, we get 7 with a remainder of 13.
Write down 7 as next digit of the result. 
Result so far: 16722.47
If we take 147 away from 160, we end up with 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Step 9:
When dividing 130 by 21, we get 6 with a remainder of 4.
Put 6 as the next digit of the answer. 
Result so far: 16722.476
If we take 126 away from 130, we end up with 4.
Since there are no more digits in the dividend, we add a zero to the remainder making it 40, and continue the process.

Moving on to step 10:
21 goes into 40 1 times with a remainder of 19.
Record the quotient 1 as the next digit in the result. 
Result so far: 16722.4761
If we take 21 away from 40, we end up with 19.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

Let's proceed to step 11:
190 divided by 21 is 9 with a remainder of 1.
Put 9 as the next digit of the answer. 
Result so far: 16722.47619
If we take 189 away from 190, we end up with 1.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

Advancing to step 12:
If we divide 10 by 21, we get 0 and a remainder of 10.
Write down 0 as next digit of the result. 
Result so far: 16722.47619
If we subtract 0 from 10, we get 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

Moving on to step 13:
21 goes into 100 4 times with a remainder of 16.
The next digit of our result is 4. 
Result so far: 16722.4761904
If we subtract 84 from 100, we get 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

On to step 14:
When dividing 160 by 21, we get 7 with a remainder of 13.
Use 7 as the next digit of our solution. 
Result so far: 16722.47619047
Subtract 147 from 160 to get 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

The final result is 16722.47619047 with a remainder of 1.3e-08.