A force of 10N extends a spring by 3cm. What will be the extension if a mass of 0.3kg is attached to the lower end of the spring?

Take g = 9.81 m/s²

F = k Δx

When F = 10 N, Δx = 3 cm :
(10 N) = k (3 cm)
k = 10/3 N/cm

When F = 0.3 × 9.81 N :
0.3 × 9.81 N = (10/3 N/cm) Δx
Δx = 0.3 × 9.81 × (3/10) cm
Extension of the spring, Δx = 0.88 cm

10N/0.03m = 1000N/3m and 0.3kg*g = 0.3*9.8 = 2.94N
So 3m/1000N * 2.94N = 8.82/1000 = 0.00882m = 0.882cm
or about 0.9cm