The series advances in a set of three as shown below (6)^1, (6+6)^1, (6+6+6)^1, (7)^2, (7+6)^2, (7+6+6)^2, (8)^3, (8+6)^3, (8+6+6)^3....... so on. The question mark corresponds to (8+6)^3 = 14^3 = 2744.

Reason Series = (x,2x,3x),((x+1)^2,(2x+1)^2,(3x+1)^2),((x+2)^3,(2x+2)^3,(3x+2)^3) Clearly x=6 Therefore the missing number = 2744