Answer
In the equation for the other wave, the sign in front of $\omega$ is minus.
Work Step by Step
We can write the expression for one of the waves:
$y(x,t) = (y_m) ~sin(k x+\omega t)$
To produce a standing wave, the two waves must be moving in opposite directions.
Therefore, in the equation for the other wave, the sign in front of $\omega$ is minus.
