The price elasticity $E$ of a product which has price $P$ given a demand $Q$ is defined as the percentage change in demand for each percentage change in price, i.e.
$$E = \frac{d \ln Q}{d \ln P}\qquad (1)$$
or alternatively
$$E = \frac{P}{Q} \frac{dQ}{dP}\qquad (2)$$
Since in your case you have
$$\ln Q = 10 - 5\ln P$$
You should be able to differentiate $\ln Q$ with respect to $\ln P$ (equation (1)) to get the result you require.
Alternatively, you could express $Q$ in terms of $P$ by exponentiating both sides of the formula, giving
$$Q = e^{10 - 5\ln P} = \frac{e^{10}}{P^5}$$
and apply equation (2) to find $E$, which may be easier if you're not that experienced with differentiation.
Once you have calculated $E$, you can decide whether the good in question is elastic or inelastic by noting that elastic goods have $|E|>1$ and inelastic goods have $|E|<1$.