nomor 21 ya, tolong dibantu, terimakasih

jawab

f(x)  = 4ax - 2b

f(5) = 20
4a(5) - 2b = 20
20a - 2b = 20 ...(i)

f(4) = 12
4a(4) - 2b = 12
16a - 2b = 12 ...(2)

(i) dan (ii)
20a - 2b = 20
16a - 2b = 12..||(-)
4a = 8
a = 2

20a - 2b = 20
20(2) - 2b = 20
40 - 2b = 20
2b = 40-20 =20
b = 10

f(x) = 4ax - 2b
f(x) = 4(2)x - 2(10)
f(x) = 8x -20

f(8) + f(9) = {8(8) -  20} + {8(9) - 20}
= (64-20) + (72 -20)
= 44 + 52
= 96