Looking at the coordinates of the points, we can see that PQ is 4 units.
Since both P and Q have the same y coordinate, we can now start to look at the x coordinate. Since they have the same y coordinate, that means that to find the length of the segment, we have to take the absolute value of the subtraction of the coordinates of the x value of each of the points. -3 - 1 is -4. The absolute value of -4 is 4, meaning that PQ is 4 units long.
