I can't remember how to work with functions\equations where there is a variable in the denominator, like

$\displaystyle

f(x)=\frac {ax+b}{cx +d} \ \mbox {or} \ y=\frac {ax+b}{cx +d}

$

specificly, how to get the inverse function after you swap the x and y, but in general just how to get the variable out of the denominator. I know you multiply by an expression equivalent to 1, but I can't remember how. This is driving me nuts. Thanks.