The perimeter of the rectangle is 40 cm. Can the length of one of its sides be equal to 21 cm?

No, none of the sides of the rectangle can be equal to 21 cm, since in this case the perimeter of the rectangle will be greater than the 40 cm given in the problem statement.This is due to the fact that the rectangle has 2 pairs of identical sides, if one side is equal 21 cm, which means the opposite side will also be 21 cm, the sum of these sides will be equal to 21 + 21 = 42 cm, which will exceed the perimeter of the rectangle, and not 1 cm of length will remain on the other 2 sides.