6*b + 14*b - 21*b + b
6*b + 14*b - 21*b + b при b = 1/4
6*(1/4) + 14*(1/4) - 21*(1/4) + (1/4)
6/4 + 14/4 - 21/4 + 1/4
0
21*b - 21*b