You can use the pythagorean theorem by making a triangle out of the points MNO. You have MN=19 and ON=15, so plug those into the theorem: a²+b²=c² where a=19 and b=15, and we are trying to find the hypotenuse, c.
19²+15²=c²
586 = c²
c = √586 ≈ 24.207
