I am going to make some assumptions here. A is the concentration of the HPO4 and B is the concentration of something else. Let's start with the equation A+B = 0.3 and subtract A from both sides:
Next, for the equation , we will multiply both sides of the equation by :
Since , we can rewrite the equation: .
This simplifies to .
So, now we have two different equations (Equation 1 and Equation 2):
The first equation tells us that wherever we see , we can replace that by because the two values are equal. So, in the second equation, we will make this replacement:
. That is where the book writes (This is why I assume that ).
Now, on the left hand side of the equation, we need to distribute the multiplication. So, we have:
This works out to (again, replace by and you have what the book wrote).
Now, we will add to both sides of the equation:
(replace by and you again have what you saw in the book).
Finally, divide both sides of the equation by 1.616 to get . Now that you have , you can plug that into (Equation 1) to get .