"-3f(x) means ""the function of x"" and is another way of writing y when the variable x is an independent variable. To get f(x) on one side of the equation, do the following:9x + 3y - 9x = 12 - 9x3y = 12 - 9x3y / 3 = (12 - 9x) / 3y = 12/3 - 9x/3y = 4 - 3xy = f(x) = -3x + 4"
