Error Analysis A gardener plans to extend the length of a rectangular garden by 7 feet. Let x represent the garden's original length. The expression 6 left parenthesis x plus 7 right parentheses represent the area of the garden, where 6 is the width, in feet, and left parenthesis x plus 7 right parentheses represent the extended length, in feet, of the garden. When asked for the area of the extended portion, the gardener incorrectly said it was 13 square feet. Expand the expression to find the area of the extension. Then describe an error the gardener might have made.