[tex]c = \frac{b}{7a-b} \ \ /\cdot(7a-b)\\\\b = c(7a-b)\\\\b = 7ac - bc\\\\b + bc = 7ac\\\\b(1+c) = 7ac \ \ /:(1+c)\\\\\boxed{b = \frac{7ac}{1+c}}[/tex]
