Select two sets of coordinates for one line. Coordinates are points through which a line runs in relation to the x-axis and y-axis. For example, you might have (0, 2) and (1, 4). The first coordinate in a set is the x-coordinate, and the second number in a set is the y-coordinate.
Subtract the second y-coordinate from the first y-coordinate and the second x-coordinate from the first x-coordinate. Place these two answers in a fraction with the y-coordinate in the numerator (on top) and the x-coordinate in the denominator (on bottom). In this example, you would subtract 4 from 2 to get -2 and 1 from 0 to get -1. Then you would write the fraction -2/-1, which simplifies to 2. This is the slope of the line.
Use the same process to find the slope of the second line.
Compare the slopes to see if the lines are perpendicular. If the second line has a slope that is the negative reciprocal of the first, the lines are perpendicular. Create a reciprocal by flipping the slope in fraction form and adding the opposite sign (positive or negative). In this case, the negative reciprocal of 2/1 would be -1/2. Therefore, if the second line had a slope of -1/2, the lines would be perpendicular.