Answer:
15ab
Explanation:
When you multiply out the square (p+q)^2, you get the trinomial ...
(p +q)^2 = p^2 + 2pq + q^2
Your binomial is (3a+2.5b), so you have ...
p = 3a
q = 2.5b
and the trinomial you get when you multiply it out is ...
(3a)^2 + 2(3a)(2.5b) + (2.5b)^2
= 9a^2 + 15ab + 6.25b^2
The first and last of these terms are shown on the right side of the given expression, so to finish out the identity, you need to replace * with 15ab.