a+1=根号下7
(a+1)^2=7
a^2+2a=6
3a^3+12a^2-6a-12
=3a^3+6a^2+6a^2-6a-12
=3a(a^2+2a)+6a^2-6a-12
=3a*6+6a^2-6a-12
=6a^2+12a-12
=6(a^2+2a)-12
=6*6-12
=24