A rancher has a square cow pen with side length \small x. He decides to change the shape of the pen while still using the same amount of fencing materials. The equation \small \left(x+10\right)\left(x-10\right)= 300 represents the relationship of the new side lengths (in feet) of the pen and its area (in square feet). If 300 represents the area of the new cow pen, what do the expressions \small \left(x+10\right) and \small \left(x-10\right) each represent?