Normally, a question such as this allows one to assume that the wall on the neighbor's garden will also serve as a wall on our garden, but the sentence ". . . and the fence must surround all four sides . . ." leads to the conclusion that the 48 feet must include the side next to the neighbor. That's waht tis assumes.
The diagram shows a rectangle, so the neighbor's length of 11 feet must be the same on the opposite side. That leaves (48 - 22) = 26 feet for the two sides. 26/2 = 13 feet each side. This leads to a garden dimesion of 11 x 13 feet.
