You may want to review equations first.
A system of equations is a list of equations, all of which have to be true. Generally, they'll have more than one letter in them. For example, y = −2 and y = 1 − 2x are one system of equations.
A solution of the system is a pair of values for x and y that makes both of the equations true. For example, a solution of the system above would be (3 ⁄ 2, −2), because when you substitute in 3 ⁄ 2 for x and −2 for y both equations are right.
Just like with one equation, the equations in a system don't have to have both letters in them. For example, in the system we used as an example, one equation didn't have an x. It would even be okay if one or both equations had no letters at all. Just substitute in the values of x and y wherever you see them. If the equations are both right, then they're a solution; if even one is wrong, then they're not.