The lower right triangle is congruent to the upper left triangle, so we have θ and 20° being the two acute angles in the triangle. The law of sines tells you ...
sin(θ)/9 = sin(20°)/5
sin(θ) = (9/5)sin(20°)
θ = arcsin(9/5·sin(20°)) ≈ 38°
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Another solution to the triangle is θ = 180° -38° = 142°. The diagram clearly shows θ as an acute angle, so we take this second solution to be extraneous.
