[SOLVED]application of differentiation

hello everyone, i have this question which i am unsure what to start with..

A pole a+b meters high stands vertically in a river b meters deep. Assume a > b. How many meters above the surface of the water must it break so that the top bending down touches the bottom of the river and the horizontal distance on the surface of the water between the points where the parts of the pole enter the water would be a maximum?

Suppose that the break point is x meters above the surface of water. Specify the function that is to be maximized in terms of x.

all i can see is the height is x + b..

any help would be appreciated.