16/21 divided by 4/9 is equivalent to 16/21 x 9/4 = 144/84 which reduces to 12/7. So 12/7 is your answer.
B. Is the answer to your question
Both millimeters and quarts are units for volume.
It would be 1 square inch. You can't make a trapezoid cause if you tried to it would turn into a square.