With this question they have given you a formula with 2 variables, they have also given you the value of the 2 variables so all that needs to be done is substitution
thus:
find: 3a2 - 7b
a = 3
b = -8
answer = 3 * (3)2 - 7*(-8)
answer = 3*9 + 56 (remember 2 negatives cancel out and give you a positive so)
answer = 27 + 56
answer = 83