sebuah rumus fungsi f(x)=4x-12. jika f(a)=8 dan f(-2)=b,maka nilai a+b adalah?? a.25 b.15 c.-15 d.-25 pakai cara yaa, tolong dibantu.

f(x) = 4x - 12
f(a) = 8
4a - 12 = 8 ... (x diganti a)
4a = 8+12
4a = 20
a = 20/4= 5

f(-2) = b
b = 4(-2) - 12
= -8-12
= -20

a + b
= 5 + (-20)
= -15 (c)