Four waves are to be sent along the same string, in the same direction: y1(x, t) = (4.00 mm) sin (2x 400t) y2(x, t)

Four waves are to be sent along the same string, in the same direction:
y1(x, t) = (4.00 mm) sin (2πx – 400πt)
y2(x, t) = (4.00 mm) sin (2πx – 400πt + 0.7π)
y3(x, t) = (4.00 mm) sin (2πx – 400πt + π)
y4(x, t) = (4.00 mm) sin (2πx – 400πt + 1.7π).
What is the amplitude of the resultant wave?