Chapter 13 - Vibrations and Waves - Learning Path Questions and Exercises - Exercises - Page 485: 23

444$\frac{N}{M}$

The formula for frequency is 1/Period. The equation for period is 2$\pi$$\sqrt{\frac{m}{k}}$. So, the equation for frequency is $\frac{1}{2\pi\sqrt{\frac{m}{k}}}$. Plugging in 5kg for m and 1.50 for frequency, after some algebra and rearranging variables, you get k is 444 N/M.