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)