And the vertical straight line l from F 1H is perpendicular to H.
Then in rt δ f 1hf2.
∠F2F 1B=30,∠F 1F2H=60,F 1H=2√3
That is, cos ∠ F2 f1h = cos 30 = f1h/f1F2.
That is f1F2 = f1h/cos30 = 2 √ 3/(√ 3/2) = 4.
That is 2c=4.