A teacher used a rectangular piece of cardboard to create an open box. The dimensions of the piece of cardboard were 25 inches by 18 inches. She cut 4 squares of equal size out of the corners of the piece of cardboard. She then folded up the edges to form the sides of the box. The area of the bottom of the box is 212.75 square inches. Let x represent the side length in inches of the square she cut from each corner. Which equation could be used to find the value of x?