I'm having trouble with this problem from a review sheet of mine.

Problem 2

Consider the price-demand equation x=f(p)=(144-2p)^(1/2) , 0>p>72.

a) Calculate the elasticity of demand at p 20, 40 and 60. Describe each case as

elastic or inelastic. For what value of p is demand unit elastic?

b) Express total revenue as a function of price p and find the value of p which

maximizes total revenue.

c) Explain, for the given price-demand equation, the connection between elasticity of

demand and the increasing/decreasing behaviour of the total revenue function

depending on price p !

What I'm really struggling with is converting the equation to the correct one for a). The answer given by the review is E=(p)/(144-2p). But the closest I've come was E=1/(144-2p)

Any help is great!