Question 748106
(1.) 5y-5x/15y^3
    5(y-x) / 5(3y^3)
     (y-x) / 3y^3

(2.) x^2-y^2/3x+3y
    ((x-y)*(x+y)) / 3(x+y)
     (x-y) / 3

(3.)divide expression (1.) by (2.)
    ((y-x) / 3y^3) * (3/(x-y))

    -(x-y) / y^3 * 1/(x-y)

    gives us -1/y^3