A classical x-y-question. We wasted 25 posts discussing what your question could mean - and it turns out that it is not even your problem. It is your attempt to solve the problem, and it is unclear if that approach can work at all.
From the problem website I guess 200 is the distance and 15 is the period. What do you expect from solutions to the equation? Note that just a few specific values of x will solve it, but a large range of speeds will work for a given traffic light.I wrote my equation to know if when the car cross the traffic light the traffic light is green or red.