The simplest way should be,
to find coefficient of in the expansion of is....
= (1 + x-2x2) can be broken as ( 1+2x) * (1-x)...
=coefficient of x4 in ((1+2x) (1-x)) 7
= (1+2x)7 (1-x)7 ( say= (A)7(B)7 )
( so we need to find coeff. of x4 from A and x0 from B and x3 from A and x from B ..........................so on till (sum of coeff. of both = 4))
= 7C4 (1)0 (2)4 * 7C0 (1)4 (-1)0 + 7C3 (1)1 (2)3 * 7C1(1)3(-1)1+ 7C2 (1)2 (2)2 * 7C2 (1)2 (-1)2 + 7C1 (1)3 (2)1 * 7C3 (1)1 (-1)3 + 7C0 (1)4 (2)0 * 7C4 (1)0 (-1)4
= 35*16 + 35 * -56 + 21*4*21 + 35*-14 +35
= -91 ANSWER.