The width of a rectangular garden is 9 feet, and its length is 12 feet. Three of these expressions equal the perimeter of the garden, in feet.

perimeter is calculated as follows
                                           
perimeter = sum of all the sides of the rectangle
                   
rectangle consists of 4 sides of which longer sides are called the length - L and shorter side is the width - W.
There are 2 lengths and they are equal and 2 widths and they are equal in size. perimeter = L + L + W + W = 2L + 2W
                                 
substituting the values in the equation
P = 2L + 2W
           
P = (2 x 12) + (2 x 9) 
P = 24 + 18 = 42 feet