We know the vertical and horizontal measures of the triangle. We use these measures as the legs in the Pythagorean Theorem.
a^2+ b^2=c^2 ⇒ 12^2+ 16^2=c^2
Finally, we can solve for the hypotenuse c. This is the distance between the points. Remember that a side length is always non-negative, so we only need to consider the positive square root.