Answer
(x+9)(x+6)
Work Step by Step
You are given $x^{2}$+15x+54.Factor it out:
$x^{2}$+9x+6x+54 -break out the b value-
($x^{2}$+9x)+(6x+54) -group the first two and last two terms-
x(x+9)+6(x+9) -Find the GCF of each grouping-
(x+6)(x+9) -factor out the common term-
=(x+6)(x+9)
