Hi, I am stuck on this question.

A ladder 8 meters long rests against a wall with its base x metres from the wall and its top y metres high. Explain why $\displaystyle x^2+y^2=64$ and differentiate the equation implicitly with respect to time. Find with three significant figures:

a) the rate at which the top is sliding down when the base is alipping out at a constant rate of 2cm/s and is 2 meters from the wall

b) the rate at which the base is slipping out when the top is slipping down at a constant rate of 2mm/s and is 7 meters high.

One part i really dont get is how to differentiate with respect to time.

Thanks