This sounds suspiciously like a homework problem, which means we are only allowed to discuss general concepts. Your professor wouldn't be best pleased if we just gave you the answer to their problems :-)
The amplitude $y$ is a function of time and distance. Since you just want the frequency the $x$ dependance doesn't matter and you can just look at the amplitude at a fixed value of $x$. I would choose $x = 0$ as the simplest option.
Now you have the amplitude as a function of just time. You have to work out how long it takes for the amplitude to go through one cycle. For example suppose you start at $t = 0$, then as you increase $t$ the value $y(t)$ will fall to zero, go negative, start rising again and eventually return to it's original value. That time is the period. Once you know the period the frequency is just the reciprocal of the period.