Answer
91.89 Hz
Work Step by Step
Here we use the equation $f=\frac{1}{2\pi}\sqrt {\frac{K}{m}}$ to find the resonant frequency.
$f=\frac{1}{2\pi}\sqrt \frac{K}{m}$ ; Let's plug known values into this equation.
$f=\frac{1}{2\pi}\sqrt {\frac{2.5\times10^{3}N/m}{7.5\times10^{-3}kg}}=91.89\space Hz$