We know that the total travel time is T = 17 h. I am going to set where the two t's are for the time out and the time back respectively.

Each portion of the trip (out and back) was 720 km.

So

and we know that

Thus

Finally we have

Thus

I leave solving this to you. I get as advertised. (Note: You may discard the negative solution as unphysical.)

-Dan