Answer:
7 inches tall
Explanation:
A graphing calculator quickly finds the solution to the cubic equation. It is convenient to define ...
f(x) = 4x^3 -28x^2 +144x -1008
so f(x) = 0 gives the value of x that is of interest. That value is x=7.
The box is 7 inches tall.
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The equation f(x) = 0 can be divided by 4 to give ...
x^3 -7x^2 +36x -252 = 0
This can be factored by grouping:
(x^3 -7x^2) +(36x -252) = 0
x^2(x -7) +36(x -7) = 0
(x -7)(x^2 +36) = 0
The only real root is x = 7, which makes a factor zero.
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Additional comment
For finding solutions to cubic and higher-degree polynomials, I find a graphing calculator to be an extremely helpful tool. Desmos provides an online calculator and apps for various platforms. It may take a little while to learn how to use it effectively, but it can save immense amounts of time and frustration. Of course, your handheld graphing calculator can be just as useful.