How do you solve -3 6-12 24-..-a7?

The numbers could be from the sequence with a(1) = -3 and the common ratio r = (-2)If so, then
a(7) = -3*(-2)^6 = -3*64 = -192.

Of course, it is entirely possible that the numbers do not form a geometric sequence but a polynomial sequence such as

a(n) = (27*n^3 - 189*n^2 + 396*n - 240)/2 and if so, a(7) = 1266.