16x(squared) + 72x + 81?
Notice the first and last terms are perfect squares. Place their square roots in the first and last positions of the binomial factors: (4x+9)(4x+9). The inner and outer products, both 36x, add up to 72x, therefore we have factored the perfect square trinomial. Can be written as (4x+9)^2 or (4x+9)squared.