jika f-1 (2x+3)= 4x+3/2x-3, maka nilai f(-7) adalah? jawabannya 5, tapi saya masih belum tau caranya

f-1(2x+3) = (4x +3)/(2x-3)
f-1(2x+3) = { 2(2x+3) -3}/ { (2x+3)-6}
f-1(x) = (2x -3)/(x -6)
f (x) = { f-1(x) }-1
misal f-1(x) = y
y = (2x -3)/(x -6)
( x -6)y = 2x -3
xy -6y = 2x -3
xy -2x = 6y -3
(y -2)x = 6y - 3
x = (6x -3)/(x -2)
maka f(x) = (6x -3)/(x -2)
f (-7) = (6.-7 -3)/(-7 -2)
= -45/-9
= 5