Add or subtract as indicated.
5x/x+3+28/x-3-90/x^2-9
I think you mean
5x/(x+3) + 28/(x-3) - 90/(x^2 - 9)
the LCD is (x+3)(x-3) or (x^2-9)
= ( 5x(x-3) + 28(x+3) - 90)/(x^2-9)
= (5x^2 - 15x + 28x + 84 -90)/(x^2 - 9)
= (5x^2 + 13x - 6)/(x^2 - 9)
= (x+3)(5x-2) /( (x+3)(x-3) )
= (5x-2)/(x-3)