11a5=5a8
11*(a1+4d)=5*(a1+7d)
11a1+44d=5a1+35d
6a1=-9d
2a1=-3d
2*(-3)=-3d
d=2
an=a1+(n-1)d
=-3+2(n-1)
=2n-5
sn=(a1+an)*n/2
=(-3+2n-5)*n/2
=n(n-4)