f(x) = 2x - 1 dan g(x) = x + 5 bila (f°g) (x) = -15, nilai x adalah

[tex](f \: o \: g)(x) = 2(x + 5) - 1 \\ - 15 = 2x + 10 - 1 \\ - 15 = 2x + 9 \\ - 2x = 9 + 15 \\ - 2x = 24 \\ x = \frac{24}{ - 2} \\ x = - 12[/tex]