central angle of circle

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find the measure of the central angle of a circle of radius 6ft that intercepted a chord of length 3.5ft.

What formula should I use for part b? I cannot use law of cosines, since I have two unknowns. Is it possible to use law of cosines? The correct answer is 33.9degrees, but how do I solve it? I plugged 3.5 for a and 6 for b and found c using Pythagorean theorem, but I get zero on the numerator. Could you please tell me if I am using the wrong formula?
 
I would draw a sketch of a circle with a cord and two radii drawn to the ends of the cord. I would label each given part. I would then draw a perpendicular from the center of the circle to the midpoint of the cord. I see that I have two right triangles with two known sides. With that information I can solve the problem. sin x = 1.75/6. Solve for x, then double it.
 
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