1)-2х(х²+14х+49)=-2х(х+7)²

2)(5х-y)²-3²=(5х-y-3)(5х-y+3)

3)y⁷(a+1)-y³(a+1)=(a+1)(y⁷-y³)=(a+1)y³(y⁴-1)=(a+1)y³(y²-1)(y²+1)=(a+1)y³(y-1)(y+1)(y²+1)