How do I do this calculus related rates problem?

[Temit96]

Water is leaking from a trough at the rate of 0.8 l/s. The trough has a trapezoidal cross section, where the width at the bottom is 55 cm, at the top is 85cm, and the height is 25 cm. The length of the trough is 3 m.
Find the rate at which the height is changing when the depth of water is 11 cm.

 

[MarkFL]

I would use cm as the unit of length and s as the unit of time. At time $t$, let the width of the water's surface in a trapezoidal cross-section be $w$. Let the depth of the water be $h$. So, the volume of water at that time is then:

$$V=\frac{h}{2}(w+55)300=150h(w+55)$$

Now, we want to express $w$ as a function of $h$. We can see it will be a linear relationship, and we know two points on the line. Can you use these two points to find the slope and then the point-slope formula to find this line?