Answer:
(2b-3)²
Explanation:
The square of a binomial is ...
(p - q)² = p² - 2pq + q²
The fact that the first and last terms are perfect squares suggests that you might want to look to see if the middle term matches this form. It does.
For p² = 4b², p=2b.
For q² = 9, q = 3.
Then 2pq = 2(2b)(3) = 12b.
So, the factoring is ...
4b² -12b +9 = (2b -3)²