To determine the capacitance when a dielectric slab is inserted, we use the formula for capacitance with a dielectric: \(C' = \frac{\epsilon \cdot A}{d - t + \frac{t}{k}}\), where \(C'\) is the new capacitance, \(\epsilon\) is the permittivity of free space, \(A\) is the area of the plates, \(d\) is the distance between plates, \(t\) is the thickness of the dielectric, and \(k\) is the dielectric constant.
The correct answer is 6.
=(3d+2d)∈0A=5d6∈0A
=56×5μF=6μF