A 2 kg block is suspended from the ceiling using a massless spring...

Given $m_1 = 2 \text{ kg}$, $k = 100 \text{ N/m}$, and taking $g = 10 \text{ m/s}^2$.

At equilibrium, the restoring force of the spring balances the weight of the block.

$k x_1 = m_1 g$

$100 x_1 = 2 \times 10$

$x_1 = 0.2 \text{ m}$

When an additional mass $m_2 = 1 \text{ kg}$ is attached, the additional restoring force balances the additional weight.

$k x_2 = m_2 g$

$100 x_2 = 1 \times 10$

$x_2 = 0.1 \text{ m}$

The initial elongation is $0.2 \text{ m}$ and the further elongation is $0.1 \text{ m}$.