(b) $x = 3\sin{\omega t} + 4\sin{(\omega t + \pi/3)}$
Comparing it with the equations
$x = r_{1}\sin{\omega t + r_{2}\sin{(\omega t + \phi)}}$
We have, $r_{1} = 3$ cm, $r_{2} = 4$cm and $\phi = \pi/3$
The amplitude of combination is
$r = \sqrt{r_{1}^{2} + r_{2}^{2} + 2r_{1}r_{2}\cos\phi}$
$= \sqrt{3^{2} + 4^{2} + 2 \times 3 \times 4 \times \cos{\pi/3}}$
$= \sqrt{37} = 6$ cm