Answer
(a) $U = 4.19 \,\text{J}$
(b) $U = 16.76\,\text{J}$
Work Step by Step
(a) The energy $U$ stored depends on the potential between the two plates. Here, the distance between the two plates is halved, therefore, the potential will be halved.
The energy is given by
$$U = \dfrac{1}{2} QV$$
Hence, when$Q$ is constant and the potential is halved, the energy storage will be halved and becomes
$$\boxed{U = 4.19 \,\text{J}}$$
(b) When $V$ is constant, the capacitance will be doubled as the distance is halved and the energy is related to the capacitance by
$$U = \dfrac{1}{2} CV^2$$
Hence, the energy stored is doubled as $C$ is doubled and becomes
$$\boxed{U = 16.76\,\text{J}}$$
