Jessie is building a cabinet door, which is pictured below. The center of the cabinet door is 12 inches by 16 inches, and the border of the cabinet door has a width of x inches. The entire cabinet door has an area of exactly 396 square inches. Clockwise from the top, the picture reads, x, 12 inches, 16 inches, x, Figure not drawn to scale.

Write an equation in standard form that can be used to find x , the width of the border.