Adding and Subtracting Polynomials

Since I do not know if by x2 you meant $2x$ or $x^2$, I'll solve for both.

Okay. here is the solution for when it is $2x$:

If the length is 5ft, then $x = 5$.

Using this to solve for $5x^3 + 4 - 2x$, we get $625 + 4 - 10 = 619$.

The perimeter is $5 + 5 + 619 + 619 = \boxed{1248}$ ft.

Now, here is the solution for when it is $x^2$.

If the length is 5ft, then $x = 5$.

Using this to solve for $5x^3 + 4 - x^2$, we get $625 + 4 - 25 = 604$.

The perimeter is $5 + 5 + 604 + 604 = \boxed{1218}$ ft.