please explain in little detail im unable to understadn it

Its just basically the use of the property of componendo and dividendo.

In question, carefully look at the LHS, you will get an idea that this property will have to be applied as the terms in numerator and denominator has just the difference of sign.

 

First consider x/3a = 2b / a+b and use this property 

Then, similarly consider x/3b = 2a / a+b and use this property

 

You will get two equations, add them to get the desired result.