Question 958124
Best to do this completely in variables and substitute the given values last.


u, the side length of the cut out squares;
w, original width
L, original length
v, volume of the open box formed
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L=2w
u=2.4
v=155
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Goal is to solve for L and w.


Folding the flaps makes a base area of the box, {{{(w-2u)(L-2u)=(w-2u)(2w-2u)}}}.


The volume will be the base area multiplied by u.
{{{(w-2u)(2w-2u)u=v}}}
This is an equation with only one unknown, being w.  The other variables are given.


Simplify that equation.
{{{(2w^2-4uw-2uw+4u^2)u-v=0}}}
{{{2uw^2-4u^2w-2u^2w+4u^3-v=0}}}
{{{highlight_green(2uw^2-5u^2w+4u^3-v=0)}}}


Instead of continuing this completely in symbols, substituting the given values NOW might make the rest of the work, either factoring if possible, or general solution for a quadratic equation, easier to do.  Your choice.  You take the rest of this.  Solve for w, and use the result to evaluate L.