A parallel plate capacitor with air between the plate has a...

Capacitance of the parallel plate capacitor when air is present between the plates, $C = \dfrac{{ε}_{0}A}{d} = 15pF$

Capacitance of the parallel plate capacitor when dielectric is filled in between the plates,

${C}^{'} = \dfrac{k{ε}_{0}A}{{d}^{'}} = \dfrac{3 . 5{ε}_{0}A}{2d} = \dfrac{3 . 5}{2} \times 15pF = \dfrac{105}{4}pF$

Therefore, $x = 105$.