Here we go! We're going to add 1258 and 1621 together.

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

Step 2: We'll start by adding the digits 5 & 2 in column 2 and get 7.

Step 3: We'll start by adding the digits 2 & 6 in column 3 and get 8.

Step 4: We'll start by adding the digits 1 & 1 in column 4 and get 2.

So, 1258 + 1621 = 2879.

