Jika f(x) =x-5 ,g(x)=x²+4 dan gof(a) =8 maka nilai a adalah..mohon dijawab

f (x) = x - 5 --> f (a) = a - 5
g (x) = x² + 4 --> g (a) = a² + 4

gof (a) = 8
g (f (a)) = 8
g (a - 5) = 8
(a - 5)² + 4 = 8
a² - 10a + 25 + 4 = 8
a² - 10a + 29 = 8
a² - 10a + 29 - 8 = 0
a² - 10a + 21 = 0
(a - 3) (a - 7) = 0
a = 3 atau a = 7

mencari (g o f)(x)
= g(f(x))
= g(x - 5)
= (x -5)^2 + 4
= x^2 -10x +25 +4
= x^2 -10x + 29

mencari nilai a
(g o f)(a) = 8
a^2 -10a + 29 = 8
a^2 -10a +29 -8 = 0
a^2 -10a +21 = 0
(a -7) (a -3) = 0
a = 7 atau a = 3