A body moves as x = 0.05 cos(10π t + π/3), with x in metres and t in seconds…

Step 1. Compare. $A = 0.05\ \text{m}$, $\omega = 10\pi\ \text{rad/s}$ and $\phi = \pi/3$.

Step 2. Frequency. We want $f$, so $f = \dfrac{\omega}{2\pi} = \dfrac{10\pi\ \text{rad/s}}{2\pi} = 5\ \text{Hz}$.

Step 3. Period. $T = \dfrac{1}{5\ \text{Hz}} = 0.2\ \text{s}$.

Step 4. Start. $x(0) = 0.05\ \text{m} \times \cos(\pi/3) = 0.05\ \text{m} \times 0.5 = 0.025\ \text{m}$.