ini caranya gimana ya kak?

19. f(x) = 7 - 4x
f(-6) = 7 - 4(-6)
f(-6) = 7 + 24
f(-6) = 31 (C)

20.
f(3) = 3a + b
3a + b = 17 ...(1)

f(-2) = -2a + b
-2a + b = -3 ...(2)
eliminasi pers. 1 dan 2
5a = 20
a = 4
3(4) + b = 17
b = 17 - 12
b = 5

f(x) = 4x + 5
f(5) = 4(5) + 5
f(5) = 20 + 5
f(5) = 25 (D)