Having obtained:

the next thing to do is collect the first three terms and write them as the square of a binomial, and also collect the last two terms, writing each of them with a denominator equal to 4a^2
We have:

.
Then, we take

to the right side and write these two terms as one:

.
Next, we take the square root of both sides, which has been shown in the solution.
Next, we have to take b/2a to the right hand side as -(b/2a), and removing the square in the denominator of the right hand side expression:

.
Answer: the steps to complete in the boxes are :

.

.

.