I need help please. Thankyou.

A bushwalker can walk at 5km/h through clear land and 3km/h through bushland. If she has to het from point a to point b following a route indicated in the image, find the value of x so that the route is covered in a minimum time. (Note: time = distance/speed)

Image: http://img294.imageshack.us/my.php?i...ntitledtp0.jpg

Please help, i need helkp getting to the answer. Please post all working. thnks