Tolong dibantu ya kakak soal No 21 pakai cara mengerjakannya juga please Terimakasih

Nomor 20
20 + 2y = 50
2y = 50 - 20
2y = 30
 y = 15

x + y = 40
x + 15 = 40
x = 40 - 15
x = 25  **E**

Nomor 21
f(x) = (9x+4) / (6x-5)
y = (9x+4) / (6x-5)
y(6x-5) = (9x+4)
6xy - 5y = 9x + 4
6xy - 9x = 5y + 4
x(6y - 9) = 5y + 4
x = (5y + 4) / (6y - 9)

Jadi f⁻¹(x) = (5x + 4) / (6x - 9)
f⁻¹(-2) = (5(-2) + 4) / (6(-2) - 9)
f⁻¹(-2) = (-10 + 4) / (-12 - 9)
f⁻¹(-2) = (-6) / (-21)
f⁻¹(-2) = 6 / 21  **C**