You can use the Pythagorean theorem to calculate the hypotenuse, since the resultant is the hypotenuse of the triangle.
[tex]r^{2} = x^{2} + y^{2}\\r^{2} = 12^{2} + 16^{2}\\r^{2} = 144 + 256\\r^{2} = 400\\r = \sqrt{400}\\r = 20N[/tex]
