The formula to find the area of a trapezoid is

It has a base that measures 8 in = a
And the other side that is parallel to base is 21 in. = b
Let the height of the trapezoid = h
The trapezoid has an area of 166.75 in²
Substituting the values we get

14.5h = 166.75
Dividing by 14.5 on both sides we get
h = 11.5 in
The height of the trapezoid is 11.5 in.