Stuck on a question, please help!

Hey guys, so I'm doing a few questions on related rates and differential approximation and I am stuck on this one, if anyone could offer ANY help at all I would be extremely grateful! Certain chemotherapy dosages depend on a patient's surface area. According to the Mosteller model

where h is the patient's height in m, M is the patient's mass in kg and S is the patient's approximate surface area in m^{2}. Assume that the patient's height is a constant 174 cm but he is on a diet. If a patient loses 3.5 kg per month, how fast is his surface area changing (in m^{2}/month) at the instant his mass is 60 kg? Give your answer correct to 4 decimal places.

**Note. You may be wondering why we have referred to the patient's ***mass* here, rather than his *weight*. Technically, a person's weight is a *force* (recall: F = Ma, where F is the force, M is the mass, and a is the acceleration, which in this case is g, the acceleration due to gravity). Force (and therefore weight) is measured in Newtons, not kilograms. Scientists, and you are one, should know that. (In orbit, an astronaut experiences *weightlessness*, not *masslessness*.)

Re: Stuck on a question, please help!

We are given:

$\displaystyle S(M)=\frac{\sqrt{hM}}{6}$

We are asked to find:

$\displaystyle \left.\frac{dS}{dt}\right|_{M=60}$

What does "a patient loses 3.5 kg per month" give us? How can we use this?