AS the capacitor is disconnected from electricity supply, the total charge of the system remains constant.

 

Q = CV₁        ---------------------- (1)

 

When we join two capacitors, they become parallelly connected.

 

So, net capacitance,

 

C_net = C + KC = (K+1)C

 

So,

 

Q = (K+1)CV₂        --------------------- (2)

 

substitute value of Q/c from one in two and you get the answer

 

 

 

Hope you have got it !!!!!!