No problem, let's work through this together. We're starting with 1721 and 7155 and adding them all up.

Step 1: We'll start by adding the digits 1 & 5 in column 1 and get 6.

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

Step 3: We'll start by adding the digits 7 & 1 in column 3 and get 8.

Step 4: We'll start by adding the digits 1 & 7 in column 4 and get 8.

So, 1721 + 7155 = 8876.

