試解

For a moment let's assume the true area value doesn't matter, and the three perpendicular lines' length are 5, 7, 8, respectively. Because the marked angles are 60 degrees, we get: (5-3.5)/x=sin60. (8-3.5)/y=sin60. That is, the edge a = x+y= 6/sin60

So, the distance between two opposite edges is b = 2a sin 60=12. 

So the perpendicular line d=12-7=5. It also represents that the true area in question is 5.