The length of the altitude from the right angle to the hypotenuse of a right triangle is the <u>geometric</u> mean of the two segments that make the hypotenuse.
First, take all the sides you already have, then add them up. (You should get 12) To find the other two sides, do some more simple addition. The 2+3+2 of the the top side add up to six (the bottom side) and sides equal each other so add another three. Total is 12+6+3=21