Answer
$x=5.4cos(8.6)t$
Work Step by Step
We know that
$\omega=\frac{2\pi}{T}$
$\implies \omega=\frac{2(3.1416)}{0.73}$
$\omega=8.6\frac{rad}{s}$
As the equation for displacement is given as:
$x=Acos\omega t$
We plug in the known values to obtain:
$x=5.4cos(8.6)t$