Yep one of the slly mistakes... BC is 57 inches but AB is 63 inches.
Remember the lenght of rectangle is 20 feet which implies 240 inches.
Half of that is 120.
Consider half the rectangle, BC + the small segment obtained by dropping a perpendicular from the point of intersection of slant line in this half with the top line..to the bottom line + the rest of the right side line = 120 inches
since x = 45 this middle segment is 6 inches.
By symmetry the first abd last segment r equal thus BC = 57 inches . From C to the end of the right side bottom vertex of the rectangle the distance is 63 inches...
This is the same as asked for by symmetry again.
I know its not so clear but that's all i could help with...without a figure.