(5a-7)(3a+1)= 5(y+2)(y+3)= -6a(a+4)(a-1)= (2a-3)(5a+7)+21= (x+4)x-(x-3)(x+7)= (5a+1)(2a-3)=(10a-3)(a+1)=

(5a-7)(3a+1)=15a²+5a-21a-7=15a²-16-7
5(y+2)(y+3)=5(y²+3y+2y+6)=5y²+15y+10y+30=5y²+25y+30
-6a(a+4)(a-1)=-6a(a²-1a+4a-4)=6a³-6a²+24a²-24a=6a³-18a²-25-24a
(2a-3)(5a+7)+21=10a²+14a-15a-21+21=10a²-1
(x+4)x-(x-3)(x+7)=x+4x+x²+7x-3x-21=9x+x²-21