He identified the location of a point on a flat suface using two numbers, written as an ordered pair (x , y) where the x number determined right/left position, and the y number up/down position. For example, the point (3 , -5) would be located three to the right and 5 down.
Let's see how good you are at identifying points located this way. Below is a Cartesian coordinate system with some points located on it. Identify each of the points, and then check your answers below.