$\omega =\sqrt{\frac{k}{m}}$

$\omega =\frac{2\pi }{T}$

$\sqrt{\frac{k}{m}}=\frac{2\pi }{T}{|}^2$

$\frac{k}{m}=\frac{4{\pi }^2}{T^2}|\cdot m$

$k=\frac{4{\pi }^{2}m}{T^2}$

$k=\frac{4\cdot {3,14}^2\cdot 0,15\text{kg}}{{\left(0,8\text{s}\right)}^2}=\frac{4\cdot 9,8596\cdot 0,15\text{kg}}{0,64{\text{s}}^2}=$  

$=\frac{5,91576\text{kg}}{0,64{\text{s}}^2}\approx 9,24\frac{\text{kg}}{{\text{s}}^2}=$  

$=9,24\text{kg}\cdot \frac{\text{m}}{{\text{s}}^2}\cdot \frac{1}{\text{m}}=9,24\frac{\text{N}}{{\text{m}}^{}}$