Let's solve this addition problem. We have 5121 and 3640, and we need to add them together.

Step 1: We'll start by adding the digits 1 & 0 in column 1 and get 1.

Step 2: We'll start by adding the digits 2 & 4 in column 2 and get 6.

Step 3: We'll start by adding the digits 1 & 6 in column 3 and get 7.

Step 4: We'll start by adding the digits 5 & 3 in column 4 and get 8.

So, 5121 + 3640 = 8761.

