The formula is 1/2 x ( a + b ) x h
A and b are the top and bottom sides and h is always the perpendicular height.
Now replace the variables...
1/2 x (6 + 14 ) x 8.4
And then...
1/2 x 20 x 8.4 which will give you 84cm^2
Hope this helps! Anymore questions let me know :)
