If a,b,ca, b, ca,b,c form an arithmetic sequence, what is the value of a3+c3−8b3a^3 + c^3 - 8b^3a3+c3−8b3 given a+b+c=15a+b+c = 15a+b+c=15 and a2+b2+c2=83a^2+b^2+c^2 = 83a2+b2+c2=83?
0
-125
125
-250