The price elasticity is given by

$\displaystyle \epsilon = -\frac{p}{q}\cdot\frac{dq}{dp}$

and the demand equation is given by

$\displaystyle q = 700-35p $

and p is given as 4 for $\displaystyle 0\leq p \leq 20 $

So i plus in 4. Which gives q as 560.

Then i take derivative of q which is 35? and derivative of p which is one?

Then throw them in the elasticity equation and get -0.25.

Does that make sense?