To find the side length from the area of an octagon and the apothem we must use the area of a polygon which is

First plug in our numbers for area and the apothem to get

Then multiply to get

Then divide both sides by 4 to get the perimeter of the figure.

When we have the perimeter of a regular polygon, to find the side length we must divide by the number of sides of the polygon, in this case 8.

After dividing we find the side length is