calc help....please!

[withy]

i'm working on some on my calc homework and i'm having a little trouble with this problem in particular:
the demand function is modeled by:

P = 740-x^1/2 , 0 <=x <= 740

find the interval on which the demand is elastic, inelastic, and of unit elasticity.

price elasticity = (p/x)/(dp/dx)

so i know that i have to substitute (740-x)^1/2 into the equation..... but how?

i e-mailed the math lab hear and they gave me this:

(((740-x)^1/2)/x)/(-1/(2/((740-x)^1/2)))

so my question to everyone is why is d/dx = (-1/(2/((740-x)^1/2)))?!?!?!?!?

its a simple problem but i jsut can't see it....