Question 19972
Like all algebraic expressions you can only "solve" it when you know a value for the unknown, in this case C.
You do not have an equation; you have an expression.
You CAN, however, simplify an expression. 
In this case it doesn't help much since the simplification results in 
{{{(5C-35-C^2)/(C^2-7C)}}} and that's it!