Okay, we are given 21 and 268. Let's add them up step by step.

Step 1: We'll start by adding the digits 1 & 8 in column 1 and get 9.

Step 2: We'll start by adding the digits 2 & 6 in column 2 and get 8.

Step 3: We'll start by adding the digits 0 & 2 in column 3 and get 2.

So, 21 + 268 = 289.

