Cool mind teaser!
-1, -6, 3, 68, 255, 654, 1379
Whats the patttern?
The sequence appears to be
[n^3 x (n-3)] +n
so
[1^3 x -2]+1 = -2+1 = -1
[2^3 x -1]+2 = -8+2 = -6
[3^3 x 0]+3 = 27x0+3 = 3
[4^3 x 1]+4 = 64x1+4 = 68
[5^3 x 2]+5 = 125x2+5 = 255
[6^3 x 3]+6 = 216x3+6 = 654
[7^3 x 4]+7 = 343x4+7 = 1379
so the next in the series is
[8^3x5]+8 = 512x5+8 = 2568
but check the maths!