Example: Figure shows three parallel plate capacitors with their capacitances. Capacitor $1$ and capacitor $2$ have the same plate areas and plate separations and capacitor $1$ is filled with air. Upper half of the capacitor $2$ is filled with a dielectric material of dielectric constant ${}_1=1\mathrm{.}5$ and lower half of it is filled with a dielectric material of dielectric constant ${}_2=2\mathrm{.}5$ as in figure. What are the

a$)$ the capacitance of the capacitor $2,$

b$)$ equivalent capacitance $C_{eq}$ and charge $q_{eq}$ of the circuit,

c$)$ the potential energy stored in capacitor $3?$