Why did you do that? How did you come to that? If you look at the formula given to you, it is not telling you to multiply 21 by 2. You multiplied the base by height and you got 21. You are right up to there. So what do you next? You simply divide by 2, that is all the formula was telling you.
The formula says to multiply the base times the height and then take half of that product to get the area.
base X height = 3 X 7 = 21
1/2 X 21 = 10.5 square units for the area.
As the formula goes....
$\displaystyle A=\frac{1}{2}bh$
$\displaystyle A=\frac{1}{2}(3)(7)$
$\displaystyle A=\frac{1}{2}(21)$
$\displaystyle A=10.5$