One to One functions

F(X) = 8*X-9

Y=8X-9
Y+9=8X
X=(Y+9)/8

F ^-1(Y) = (Y+9) /8

simple test:
F( 2 ) = 8*2-9 = 7
F ^-1( 7 ) = (9+7)/8 = 16/8 = 2