Homework help with finite series please.

The product of three consecutive terms in a geometric sequence is -1000, and their sum is 15. Find the common ratio.

x/r , x , xr = three consecutive terms in a geometric sequence
(x/r )(x ) (xr ) = -1000 ---> x = -10
so
-10/r - 10 -10r = 15 , solve this equation for the common ratio r