How to derive this equation ?

Welcome to PF;
It is impossible to derive the equation from the information (none) given.
We need to know what the equation is supposed to represent and what all the letters mean.

It looks kinda like a coupled-mass relationship, where one mass is hanging off a frictionless surface, and you are solving for the tension in the string connecting them.
That's just a guess - don't make people guess what you are asking. If the guess is good, then you start by drawing free-body diagrams for the masses like you were taught in school and apply Newton's laws.

 

Just like I said - draw free body diagrams and apply Newton's laws of motion.