Form it into a right triangle with MN being the hypotenuse (side opposite the right angle)
To make this a right triangle, plot a point at (8,-2) and connect the sides. The lengths of the legs that form the right angle are both 6 units. Use the Pythagorean theorem to solve for length of MN. a^2+b^2=c^2. (a and b are the legs that form the right angle and c is the hypotenuse)