Chapter 3 - Exponents, Polynomials and Functions - 3.5 Special Factoring Techniques - 3.5 Exercises - Page 279: 41

$(\sqrt H+3)^2$

The first term is $\sqrt H$ squared, the last term is $3$ squared, and the middle term is twice the product of these two terms. Now we can factor the trinomial into a binomial squared. $H+6\sqrt H+9$ $=(\sqrt H+3)(\sqrt H+3)$ $=(\sqrt H+3)^2$