For a rectangle, area = length × breadth. So length = area ÷ breadth = 252 ÷ 6 = 42. The missing length is 42 mm. Notice that both measurements are lengths, but the answer is in square units because we are counting how many unit squares cover the surface. Do not add the sides here; adding sides is used for perimeter, not area. So the correct choice is 42 mm.