Diketahui f(3x-3) = 13-6x. Nilai dari f(-1) adalah

f(3x-3) = 13-6x
f(3x-3) = 7+6-6x
f(3x-3) = 7-6x+6
f(3x-3) = 7-2(3x-3)
==> f(x) = 7-2x
Jadi, f(-1)= 7-2(-1) = 7+2 = 9

Misalkan
3x - 3 = a
x = (a + 3)/3

f(3x - 3) = 13 - 6x
f(a) = 13 - 6((a + 3)/3)
f(a) = 13 - 2(a + 3)
f(a) = 13 - 2a - 6
f(a) = 7 - 2a
f(x) = 7 - 2x

f(-1) = 7 - 2(-1)
f(-1) = 7 + 2
f(-1) = 9