To solve this, you need to isolate/get the variable "a" by itself in the equation:
-8a + 32 = 72 Subtract 32 on both sides
-8a + 32 - 32 = 72 - 32
-8a = 40 Divide -8 on both sides to get "a" by itself
a = -5
PROOF
-8a + 32 = 72 Substitute/plug in -5 into "a" since a = -5
-8(-5) + 32 = 72 (two negative signs cancel each other out and become positive)
40 + 32 = 72
72 = 72