(1−x−x2+x3)6=[(1−x)(1−x2)]6 =(1−x)12(1+x)6 In the expansion of (1−x)12, coefficient are of the form (−1)r12Cr and in (1+x)12, coefficient are of the form ‌6Cr Coefficient of x4 in expansion of (1−x−x2+x3)6=‌12C0×‌6C4−‌12C1×‌6C3+‌12C2×‌6C2 −‌12C3×‌6C1+‌12C4×‌6C0 =15−240+990−1320+495=−60