tolong bantu jawab menggunakan cara ya!!

(fog)(a) = f (g(a)) = 5 g (a)^2 + 1 =21
maka, 5 g(a)^2 = 21 - 1 = 20
g(a)^2 = 20/5 = 4
g (a) = akar (4)
g(a) = +2 atau -2
1/a + 1 = +2 atau -2
* 1/a + 1 = 2
1/a = 2-1
1/a = 1
a = 1
*1/a + 1 = -2
1/a = -2-1
1/a = -3
a = - 1/3