Объяснение:
1)21a²-(12a-5+21a²)=21a²-12a+5-21a²=-12a+5
2)(x²+x-1)-(x²-x+1)=x²+x-1-x²+x-1=2x-2
3)-7x²+x+(x+6x²)=-7x²+x+x+6x²=-x²+2x
4)(12-p²)+(p³+2p²-p+15=p³-3p²-p+27
