${y_1} = 2a\sin (\omega t - kx)$ and ${y_2} = 2a\sin (\omega t - kx - \theta )$
The amplitude of the medium particle will be
${y_1} = {10^{ - 6}}\sin [100\,t + (x/50) + 0.5]m$
${y_2} = {10^{ - 6}}\cos \,[100\,t + (x/50)]m$
where $ x$ is expressed in metres and $t$ is expressed in seconds, is approximately .... $ rad$