An incident wave y = -4 sin (3p t - 6p x) and a reflected wave y = 4 sin (3p t + 6p x) produce a standing wave.
What is its equation?

Remember the standard form:
y1 = A sin(w t - kx)
y2 = -A sin(w t + kx)
Produces:
y = y1 + y2 = -2Acos wt cos kx

In this case the amplitudes are reversed in sign but not the direction of the waves so only the sign of the standing wave amplitude is reversed.
y = 8 cos wt sin kx

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