For each triangle, its area is half of the product of the lengths of any its two sides and the sine of the angle between them. In particular, for your triangle PQR its area =, or 18 = , 18 = 24*sin(R). It gives sin(R) = = = 0.75. Therefore, R may have two and only two values 1) R = arcsin(0.75) (acute angle), and 2) R = 180° - arcsin(0.75) (obtuse angle)