由a^2+3a=3可知,a≠0.故等式a^2+3a=3两边同乘以a,可得a^3+3a^2=3a.===>a^3=3a-3a^2.又a^2+3a=3.故原式=2a^3+8a^2+6=2[3a-3a^2]+8a^2+6=6a-6a^2+8a^2+6=2a^2+6a+6=2[a^2+3a]+6=2*3+6=12.===>原式=12.