Halliday, Resnick & Walker — Waves–I: Problem Problem_36

Four waves are to be sent along the same string, in the same direction: \[y_1(x, t) = (4.00 \text{ mm}) \sin(2\pi x - 400\pi t)\] \[y_2(x, t) = (4.00 \text{ mm}) \sin(2\pi x - 400\pi t + 0.7\pi)\] \[y_3(x, t) = (4.00 \text{ mm}) \sin(2\pi x - 400\pi t + \pi)\] \[y_4(x, t) = (4.00 \text{ mm}) \sin(2\pi x - 400\pi t + 1.7\pi).\] What is the amplitude of the resultant wave?