Let
x-------> the length side of the original cube
we have

Divide by
both sides

The system of equations is equal to
--------> equation 
--------> equation 
using a graphing tool
see the attached figure
we know that
the solution of the system of equations is the intersection both graphs
therefore
the solution is

therefore
<u>the answer is</u>

-2x=5 is the right answer
The questions is asking which one and i say is graph B