You would use the Pythagorean Thereon to find the answer.
+ = You can You can draw a right triangle from the two points. One way you could do this is draw a line straight down from N and stop when you get down to N'. Then continue drawing the line horizontally until it gets to N'. Connect N' to N and will have drawn a right triangle.
The vertical height of your triangle would be the difference of you y values and the horizontal distance would be the difference between your x values.