Find E(x) = - (x * D'(x)) / D(x)

q = D(x) = √ (300-2x)
D'(x) = -1 / √ (300-2x)

right?
So

E(x) = - (x * D'(x)) / D(x) = - x * x (-1/ √ (300-2x)) / √ (300-2x)
= x / √ (300-2x)

right?

E(x) = - x*( -1 / √ (300-2x))/(√ (300-2x)) = x/(300 - 2*x), answer!