I keep getting a negative for the surface area. Can you please explain?

11. [0.33/1 Points] DETAILS PREVIOUS ANSWERS SCALCET8 14.5.039. MY NOTES The length f, width w, and height h of a box change with time. At a certain instant the dimensions are f = 2 m and w = h = 5 m, and f and w are increasing at a rate of 1 m/s while h is decreasing at a rate of 3 m/s. At that instant find the rates at which the following quantities are changing. (a) The volume. 5 m /s (b) The surface area. 8 x m2 /s (c) The length of a diagonal. (Round your answer to two decimal places.) m/s