(a) $x_{1} = A\sin{(\omega t + \varnothing_{1})}$
$x_{2} = A\sin{(\omega t + \varnothing_{2})}$
$x_{2} - x_{1} = A\lbrack\sin\left( \omega t + \varnothing_{2} \right) - \sin\left( \omega t + \varnothing_{1} \right)\rbrack$
$= 2\ A\cos{\left( \frac{2\omega t + \varnothing_{1} + \varnothing_{2}}{2} \right)\sin\left( \frac{\varnothing_{2} - \varnothing_{1}}{2} \right)}$
The resultant motion cam be treated as a simple harmonic motion with amplitude 2$A$ $\sin\left( \frac{\varnothing_{2} - \varnothing_{1}}{2} \right)$
Given, maximum distance between the particles= $x_{0} + A$
$\therefore$ Amplitude of resultant SHM
= $x_{0} + A - x_{0} = A$
= $2\ A\ \sin\left( \frac{\varnothing_{2} - \varnothing_{1}}{2} \right) = A$
$\varnothing_{2} - \varnothing_{1} = \frac{\pi}{3}$