Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
What is the capacitance of the third capacitor in the system below, if we need the net equivalent capacitance to be $4\;\rm \mu F$?
$$\color{blue}{\bf [b]}$$
Solving the given formula for $C$,
$$C=4-2=\color{red}{\bf 2}\;\rm \mu F$$
