You can use variables (operands) in the built-in Math Solver if they conform to the naming conventions as follows

 The characters in a variable name must be a-zA-Z, or 0-9
 The first character must not be a number
 Variable names must not be Constants ("E" or "PI") 
 Variable names must not be Functions ("sqrt", "sin", "cos"). 
 Variable names must not be Operators ("+", "-", "*", "/").