Continuous random variables — practice question

The diagram illustrates the probability density function for a random variable $X$. For $x = 0$ to $x = a$, the graph is a straight line passing through $O$ with gradient $k$, where $k$ and $a$ are positive constants. For all other values of $x$, $f(x) = 0$. The median of $X$ is given as $\sqrt{2}$.