Hi!
The equation to find the area of a triangle is...
(a = area
b = base
h = height)

Fill in the values that we already have.
Divide both sides by 1/2
1722 = 82*h
Divide both sides by 82.1722/82 = 82/82*h
21 = h
The answer is B. 21 cmHope this helps! :)