Answer
$k = 1.29\times 10^5~N/m$
Work Step by Step
Each spring oscillates with a mass of $\frac{1450~kg}{4}$ which is $362.5~kg$
We can find the spring constant of each spring:
$f = \frac{1}{2\pi}~\sqrt{\frac{k}{m}}$
$2\pi~f = \sqrt{\frac{k}{m}}$
$4\pi^2~f^2 = \frac{k}{m}$
$k = 4\pi^2~f^2~m$
$k = (4\pi^2)~(3.00~Hz)^2~(362.5~kg)$
$k = 1.29\times 10^5~N/m$
