For this one we do not have a right triangle. We have an angle of

We can use the law of cosines.

Draw the diagram. It'll help.

Let be the rate of one biker and

be the rate of the other.

In two hours, one biker will have traveled 15*2=30 km

and the other will have traveled 20*2=40 km

So, we have a whole bunch of info to enter into the law of cosines formula after it is differentiated.

We need dc/dt.

Use the law of cosines to find c with a=30, b=40, theta=Pi/3

Remember, the bikers maintain a constant angle, so . That makes it easier.

Now, go for it. You have all you need. If you get hung up, let me know.