The function of demand is:\begin{align*} D(p) = 66-3p-p^2 \\\end{align*} The function of supply is: \begin{align*}S(p) = 4p^2+8p-114\\ \end{align*} The task is to find price elasticity of demand in the point of economic equilibrium.
I have found out that the equilibrium price is 5 and equilibrium demand is 26.
I also have a formula that states that $E = k * P/Q$ , where $P$ - equilibrium price, $Q$ - equilibrium demand and $k$ - coefficient of $S(p)$ slope
How to find $k$ or are there another methods to solve this task?