Question 921759
Language skills are important:
<i>The width is <s>3x</s> three times its height, and its length is 11 inches more than its height.</i>


Length = h+11
Width = 3h
Height =  h


Volume of the box given as 720 cubic inches.
L*W*H=VOLUME


{{{highlight_green((h+11)(3h)h=720)}}}


{{{3h^2(h+11)-720=0}}}
{{{3h^3+33h^2-720=0}}}
{{{h^3-11h^2-240=0}}}


You could use Rational Roots theorem and synthetic division...
but a quick check in google search engine's graphing capability shows {{{h=12.5}}}