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-z, A-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 ("+", "-", "*", "/").
