The given triangle is a right triangle.
The tangent of an angle of right triangle is obtained by dividing the opposite side of the angle with the adjacent side of the angle.
The opposite side of angle R is 5 cm and the adjacent side of angle R is 12 cm.
tan R = 5 / 12 = 0.4167
R = arctan (0.4167) = 22.6 degrees.
