How do you make f the subject of the following a = c/d + e/f

1)Multiply out the right hand side to give a common denominator: a= (Cf+de)/df2) Cross multiply to get rid of the denominator fda = cf + de3) bring all terms containing f to the same side and factorise fda - cf = de / f (da-c) = de4) divide to make f the subject F = de/(da-c)