math

fx / gx   =   (x-3)(x+3)  /  (x-3)(x-4)         factored

     so we can see that x cannot equal 3 or 4   because the denomintaor would be ZERO....undefined/not allowed

 

Now we can 'cancel out' the x-3 in the numerator and the denominator andit becomes

(x+3)/(x-4)  and    x cannot be 3 or 4