how to solve this numerical ?

Recall that V = Ed

For the capacitor with plates whose separation is increased by 2mm, the potential difference can be written as

V = E₀d + (E₀/K)x   -----(1)

Here d = 4mm the original distance between the plates. x = 2mm the further increase in distance.

E₀ = V/d , in the question both V and d are given so find E₀

Use this value in eq.1 with V = 100V and putting in all other values find K