In a+3(5a+8)=72, how do you find a?

Question

I know its three, but im not aloud to guess and check... I can't exactly figure out how to work it backwards.

Answer 1

a+3(5a+8) = 72

a+15a+24 = 72

Subtract 24 from each side.

16a = 48

Divide to each side by 16.

a = 3

Check

a+3(5a+8) = 3+3(15+8) = 3+3(23) = 72.