Answer
$5.6 \times 10^2 N/m$.
Work Step by Step
Find the spring constant by dividing the magnitude of applied force by the displacement.
An force of 210N-75N = 135 N force caused a displacement of 24 cm.
$$k=\frac{F}{x}=\frac{135N}{0.24m}\approx 5.6 \times 10^2 N/m$$
