Two sinusoidal waves of the same period, with amplitudes of 5.0 and 7.0 mm, travel in the

Question:

Two sinusoidal waves of the same period, with amplitudes of 5.0 and 7.0 mm, travel in the same direction along a stretched string; they produce a resultant wave with an amplitude of 9.0 mm. The phase constant of the 5.0 mm wave is 0. What is the phase constant of the 7 .0 mm wave?