s4=s9 a1=-12
4(a1+a4)/2=9(a1+a9)/2
4(a1+a1+3d)=9(a1+a1+8d)
10a1+60d=0
d=2
Sn=n(a1+an)/2=n(a1+a1+(n-1)d)/2=n(a1+n-1)=n(n-13)