y = -6 cos 6t sin 4x

w = 6, k = 4

Reversing the direction of the waves but not the sign of the amplitudes (i.e., incident + and reflected -)
would reverse the sign of the standing wave amplitude so it would be 2 X 3 = 6.

Since the standing wave amplitude is negative, then the signs of the travelling wave amplitudes must also be reversed.

y_{i} = -3 sin (6t + 4x) (+ sign means to the -x direction)

y_{r} = 3 sin (6t - 4x) (- sign means to the +x direction)