Answer
$s(t) = 0.21~cos~55\pi t$
We can see a sketch of the graph of $s(t)$ in the window $[0,0.05]$ by $[-0.3, 0.3]$
Work Step by Step
$s(t) = a~cos~\omega t$
$s(0) = 0.21$ and it is given in the question that $s(0)$ is the maximum displacement. Therefore, $a = 0.21$
We can find $\omega$:
$\omega = 2\pi~F = (2\pi)(27.5) = 55\pi$
We can write the expression for $s(t)$:
$s(t) = 0.21~cos~55\pi t$
We can see a sketch of the graph of $s(t)$ in the window $[0,0.05]$ by $[-0.3, 0.3]$
