No units are shown on the angle, so we assume it is in radians. Then the distance from the sun to the star is
... distance = (1 AU)/tan(0.000302/2) ≈ 6,623 AU
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The tangent is the ratio of the opposite to adjacent sides of a right triangle. If we draw a line from the star to the sun, it bisects the angle and makes two right triangles. The distance of interest is the adjacent leg, and 1 AU is the opposite leg. Hence we have
... tan(0.000302/2) = (1 AU)/(distance)
Rearranging this equation gives the one above.
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For angles this small, the tangent is very nearly equal to the angle, so the distance is essentially (1 AU)/0.000151 ≈ 6623 AU