Nomor 54 dan 55 . Nmr 55 jawabannya 12-4x . Tpi nggak tahu caranya . Mohon bantuannya

55) Rumus : f(x) = y maka f^-1(y) = x
f(2x + 4) = 2 - x/2 => f^-1(2 - x/2) = 2x + 4
Misal : a = 2 - x/2 => x/2 = 2 - a => x = 4 - 2a
f^-1(2 - x/2) = 2x + 4
f^-1(a) = 2(4 - 2a) + 4
f^-1(a) = 8 - 4a + 4
f^-1(a) = 12 - 4a
f^-1(x) = 12 - 4x