These examples demonstrate substitution in its simplest form. We may also be given problems that are a little more challenging. We may, for example, be given two linear equations that each contain two unknown variables. In mathematics, we frequently call such combinations "simultaneous equations." We could show each of these equations in the form ax + by = c. These two simultaneous equations each have the same form, and differ only by the values of their constants. We will presently learn that with two equations such as this, there is generally enough information to solve individually for the values of both x and y.