I have to find the derivative of sqrt.(4-x^2), top half of a circle, at the point x=-1.

My teacher won't let me use the rules of differentiation. so I can't just take the 1st derivative and plug -1 in. (I found the 1st derivative so I know the answer is 1/sqrt.3 or .5773)

Is there any other way to find the derivative at that point? If so could you explain how to do it.

Thanks in advance.