21
15
You need to add 6 - (-9) = 6 + 9 = 15.
how much do you need to add to 15 to make it 60
12+9=21 21+15=36 36+13=49 49+6=55
15
9 - (-6) = 9 + 6 = 15
6
You need to add 12.
30
-3
15
They all relate by adding up the 6+6+3=15 You can not get to the sum of 15 is you don't have the first 3 numbers to add up.