smc

The resulting metal piece looks something like this where the white parts are squares of length $x$: From here, try to visualize the rectangular prism coming together and realize the height is $x$, the length is $14-2x$, and the width is $10-2x$. Therefore, the volume is $x(14-2x)(10-2x)=x(4x^2-48x+40)=\boxed{140x-48x^2+4x^3}$.