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.