Let h be the length of the altitude from C to AB.
Since ∠BCO=20∘, △BCO is a 30-60-90 triangle. Therefore, BC=3CO=3h=43.
Since all the side lengths in the rhombus are equal to 12, we have that AB=43. Therefore, the area of the rhombus is 21⋅AB⋅h=21⋅43⋅43=48.
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