I need help with this.... Can anyone help me??? Many thanks

The figure shows a circular field with centre O, and radius 2 km. AC is the diameter of the circle, and B lies on the circumference. Jack cycles along the straight line from A to B at a speed of 12 km/hr. He then walks along the straight line from Bto C at a speed of 5 km/hr. Given that the distance from B to C is x km, and the total time taken is T hours, find the value of x so that Jack can complete his journey in the shortest time possible.