2d=a-b/b-c - solve for a
\(2d=\frac{a-b}{b-c}\\\\\frac{a-b}{b-c}=2d\ \ \ \ \ |multiply\ both\ sides\ by\ (b-c)\\\\a-b=2d(b-c)\ \ \ \ |add\ "b"\ to\ both\ sides\\\\\boxed{a=2bd-2cd+b}\)