7х-4(х-2(х-5(х-6)))=21
7х-4(х-2(х-5(х-6)))=21,7х - 4х + 8 (х - 5 (х - 6)) = 21, 7х - 4х + 8х - 40 (х - 6 ) = 21, 7х - 4х + 8х - 40х + 240 = 21, - 29 х = - 240 + 21, -29х = -219. х= 219/29 х=7 целых 16/29