f(x) = (4x-1) / (2x+3)

y = f(x) = (4x-1) / (2x+3)
y(2x+3) = 4x-1
2xy+3y-4x+1 = 0
x(2y-4)+3y+1 = 0
x(2y-4) = -3y-1
x = -(3y+1) / (2y-4)

finv(x) = -(3x+1) / (2x-4)