the sum of the first ten tems of a linear sequence is -60 and the sum of the first fifteenth term of the sequence
You really should proofread what you post:
and the sum of the first fifteen terms of the sequence ... is what?
In any case,
Sn = n/2 (2a + (n-1)d)
so
10/2 (2a+9d) = -60
15/2 (2a+14d) = ???
Finish your other equation and then solve for whatever you need.