помогите срочно!! выполнить вычитание

1) (x-2)/3x² - (7-x)/x³ =

= (x²-2x+3x-21)/3x³ = (x²+x-21)/3x³

2) 36/(a²+3a) - 12/a = (36-12a-36)/a(a+3) =

= - 12/a+3

3) a²/(a²-25) - a/(a-5) = (a²-(a+5)a)/(a²-25) =

= (a²-a²-5a)/(a²-25) = - 5a/(a²-25)

4) 20x²/(4x-7) - 5x = (20x²-20x²+35x)/(4x-7) =

= 35x/(4x-7)