Write down the coordinates of the line's endpoints. The points will each have an x coordinate and a y coordinate. For example, you might have a line with the following endpoints: (1, 2) and (4, 6). The first coordinate is the x coordinate, and the second coordinate is the y coordinate.
Subtract the first x coordinate from the second x coordinate and square your answer. In this example, you would subtract 1 from 4 to get 3. Then you would multiply 3 times 3 to get 9.
Subtract the second y coordinate from the first y coordinate and square your answer. In this case, you would subtract 2 from 6 to get 4. Then multiply 4 times 4 to get 16.
Add your answers from steps two and three. In this example, you would add 16 and 9 to get 25.
Take the square root of your answer from step four to determine the true length of the line. In this case, you would take the square root of 25 to get 5 (since 5 times 5 equals 25). Therefore the length of the line would be 5 units.