29.26 inches.
Since the object Robbie drew is a square, we know that all 4 sides are the same length and the area of the square is L^2, where L is the length of one side. So
A = L^2
856 in^2 = L^2
sqrt(856 in^2) = l&2
29.26 in = L
So the length of one side is approximately 29.26 inches.
