No problem, let's work through this together. We're starting with 5121 and 1505 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 & 0 in column 2 and get 2.

Step 3: We'll start by adding the digits 1 & 5 in column 3 and get 6.

Step 4: We'll start by adding the digits 5 & 1 in column 4 and get 6.

So, 5121 + 1505 = 6626.

