Ok, so
Let me draw the situation here below:
Ok, so, as You can see, the first thing we did was to sum the similar terms.
Now,
Ok, now, as You can see, we ask for a term which that if we multiply by it, the term on the other side, could be canceled. And, we multiply by all the expression divided.
Now, We do the same thing.
Finally,
And finally, the result is:
