Newton’s Third Law of Motion states that for every action there is an equal and opposite reaction. So look for a scenario in which something had force applied upon it and the reaction is a force in the opposite direction of the same size.
Use the Pythagorean theorem to find the length of the diagonal, or the hypotenuse of an imaginary triangle. 30^2 + 40^2 = 2500, which is 50^2. So, the magnitude is 50.
