the Garden only is 26ft by 51ft, however is it surround by 3ft wide path. so if you draw it you can see that the length of the garden is 51ft the path add 3ft to each side so the length will become 51 + 3 + 3 = 57 ft same thing for the width we add 3ft to each side. at the end we calculate the area from the new Length and Width and minus it from the area of the garden.
