To find the force on the block, use Newton #1: F = m A
Force = (2.3 kg) x (0.27 m/s²)
Force = (2.3 x 0.27) kg-m/s²
Force = 0.621 Newton
The spring stretches 1 meter when to pull with 18 N of force.
To pull with only 0.621 N of force, it only has to stretch (0.621/18) of a meter. That's 0.0345 meter, or 3.45 cm .