The velocity of the sound is generally greater in solids than in gases at S.T.P - why?

What do you understand by a wave? What are transverse and longitudinal waves? 

Show that a progressive wave of amplitude $a$ and velocity $v$ moving in the positive $x$ direction is represented by the equation, $y=a\sin \frac{2\mathrm{\pi }}{\lambda }\left(vt-x\right)$. How will the wave be represented if it moves in the opposite direction ?

Mention some practical applications of multiple reflections of sound.