a3=-23, a8=-8, d-?, a1-?
a3=a1+2d => a1+2d=-23
a8=a1+7d => a1+7d=-8
2d-7d=-23+8
-5d=-25
d=25/5
d=5
an=a1+(n-1)*d
-23=a1+(3-1)*5
-23=a1+10
-a1=10+23
a1=-33
