To solve this problem, we need to use Hooke's Law.
Hooke's Law:
F = -kx
where F = mg
m = mass of the object
g = gravitational force
k = spring constant
x = length of stretch of the spring
Solve for the spring constant, k:
2.0 kg * 9.81 m/s^2 = -k*(6.1cm/100cm/m)
k = 321.64
The stretch of the spring with mass of 2.5 kg is then determined:
2.5 kg * 9.81 m/s^2 = -321.64 *(x)
x = 7.62 cm