3a^2-{5a(4a-1)-7a^2}
=3a^2-(20a^2-5a-7a^2)
=3a^2-(13a^2-5a)
=-10a^2+5a3a^2-{5a-(4a-1)-7a^2}3a^2-{5a-(4a-1)-7a^2}=3a^2-(5a-4a+1-7a^2)=3a^2-(a+1-7a^2)=3a^2-a-1+7a^2=10a^2-a-1