Elasticity of Demand problem

Can someone please help me out with this. I know the general formula is:

E (p) = -f'(p) x P / f (p)

The question says: Compute the elasticity of demand and determien whether it is elastic (>1) , inelastic (<1) or unitary (1) at the given unit price p0. if the unit price is increased slightly from po, will revenue increase or decrease?

1. x = -4/3p + 24; p0 = 6

I'm not sure how to plug this in into the formula. I tried a few ways out but coudln't get it to work. Thanks for the help in advance.