Calculating Price Elasticity of Demand
The answer will vary slightly depending on which notion of elasticity you're using. 

Arc elasticity (or midpoint elasticity) uses the formula
\begin{equation}
\epsilon^\text{arc}=\frac{Q_1-Q_0}{P_1-P_0}\cdot\frac{\frac12(Q_0+Q_1)}{\frac12(P_0+P_1)},
\end{equation}
where $\frac12(Q_0+Q_1)$ is the midpoint between $Q_0$ and $Q_1$. Note that in your case, $7.5$ is the midpoint between $7$ and $8$. 

Point elasticity uses the formula
\begin{equation}
\epsilon^\text{point}=\frac{\mathrm dQ}{\mathrm dP}\cdot\frac{P}{Q}.
\end{equation}
Here, $\frac{\mathrm dQ}{\mathrm dP}$ is the derivative of the demand function (evaluated at the point you want to calculate the elasticity, but it will be constant if demand is linear), and in your case, $P=7.5$ and $Q$ is the quantity demanded corresponding to that price.