Assuming that these are all sides of a right triangle, i would make sense if AB and BC were legs of a triangle, with the AC being the hypotenuse. Given these, we can use the pythagorean theorem to solve for the missing length. This is shown below:
AB^2 + BC^2 = AC^2
40^2 + BC^2 = 50^2
BC^2 = 50^2 - 40^2
BC^2 = 2500 - 1600
BC = sqrt(900)
BC = 30
Therefore, BC is 30 feet long.