a2+a5+a8=9
3a5=9
a5=3
a3a5a7=-21
a3a7=-7
(a5-2d)(a5+2d)=-7
a5^2-4d^2=-7
9-4d^2=-7
d^2=4
d=±2
a11=a5+6d=3+12=15
a11=a5+6d=3-12=-9