Hi there!
We can use Newton's Second Law to solve.
Recall the force from a spring:
[tex]F = -k\Delta x[/tex]
F = Force (N)
k = Spring constant (N/m)
Δx = displacement from equilibrium (m)
Sum the forces acting on the mass. Weight (Mg) acts downward, while the spring's force is upward.
[tex]\Sigma F_{y} = W - k\Delta x[/tex]
Since the mass is in equilibrium at this point, ∑F = 0 N.
[tex]0 = W- k\Delta x\\
\\
k\Delta x = W[/tex]
Plug in the givens to solve for weight:
[tex]100(0.65 - 0.50) = \boxed{15 N}[/tex]
