Remember that the formula for the perimeter of a rectangle is:
![2w + 2l](https://img.qammunity.org/2019/formulas/mathematics/middle-school/5o61w7ycpa4dq11mkl1dpy4svs76s0ak3a.png)
is the width of the rectangle
is the length of the rectangle
Thus, in this case, you can see that we are adding two of the same monomials and two of another monomial. One monomial is
and another monomial is
. In this case, these monomials would fulfill the perimeter equation of a rectangle.
Applying this perimeter structure, we can find that this expression represents a perimeter equation of a rectangle with sides x + 15 and x + 5.