-(-7a^2-5a+2a^2-3a+2a)-4a
=7a^2+5a-2a^2+3a-2a-4a
=5a^2+2a后面是4a平方= =-(-7a^2-5a+2a^2-3a+2a)-4a^2=7a^2+5a-2a^2+3a-2a-4a^2=a^2+6a