If $x - y = 6$ and $x + y = 12$, what is the value of $x$?
This is a system of two linear equations with two variables, $x$ and $y$. One way to solve such a system is to add or subtract the equations to eliminate one variable and solve for the other. In this case, if I add the equations, I get $2x = 18$, which simplifies to $x = 9$. This is the value of $x$ that satisfies both equations. 