16 kg.
Hooke's law is:
F = kX
where
F = Force
X = displacement of spring
k = force constant for spring.
So let's calculate F
F = kX
F = 525 N/m * 0.30m
F = 157.5 N
F = 157.5 kg*m/s^2
Now that we know the force, we can calculate the mass by dividing by the gravitational acceleration which has been given to use as 10 m/s^2. So 157.5 kg*m/s^2 / 10 m/s^2 = 15.75 kg
So the mass of the crate is 15.75 kg. Rounding to 2 significant figures gives 16 kg.