→a*→b=(2→e1+→e2)*(-3→e1+2→e2)=-6→e1^2+2→e2^2+→e1→e2
=-6+2+1*1*cos60=-7/2
cosα=→a*→b/(|→a|*|→b|)
|→a|=√(→a)^2=√(4→e1^2+→e2^2+4→e1*→e2)=√7
|→b|=√(→b)^2=√(9→e1^2+4→e2^2-12→e1*→e2)=√7
∴cosα=-1/2
∴α=120°