Density of $Y_a = sup_{t \in [\tau, \tau + a]}B_t$, where $B_t$ is a Brownian Motion

Question

Consider Brownian Motion $(B_t, t \geq 0)$ and $\tau = inf\left\{ t: B_t = x \right\} $ for $x > 0$. Find density of $Y_a = sup_{t \in [\tau, \tau + a]}B_t$. Could anybody give a hint how to approach this problem? Thanks a lot for any help!