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.