Ummm... as a general rule, it's not possible to "solve a single equation with two unknowns" except for special cases where one of the unknowns can be eliminated. What you're trying to solve is "a system of simultaneous equations with x unknowns". In general, you need a system of x equations in order to solve for x unknowns. If the equations are linear, then this subdomain of mathematics is known as linear algebra or linear programming. If the equations are non-linear, then you need to work with systems of differential equations in order to solve for the unknowns.

With two unknowns