[tex]l = \frac{2b+c}{2+a} \ \ /\cdot(2+a)\\\\l(2+a) = 2b+c\\\\2l + al = 2b+c\\\\al = 2b+c - 2l \ \ /:l\\\\a = \frac{2b+c-2l}{l}\\\\a = \frac{2b+c}{l} - 2[/tex]
