There is 21 square units by 3 and 7. 3 as the column and 7 as a row. You can solve it by multiplying 3 with 7 which gives you 21 or adding 7+7+7 or 3+3+3+3+3+3+3 and all three gives you 21 as the answer. So the square unit of 3 and 7 is 21.
