For this problem, we apply the Angle Addition Postulate. This postulate states that the sum of all the interior angles is equal to the total angle it forms. Looking at the attached figure, this means that ∠RSU = ∠RST + ∠TSU. So, to find the value of angle RSU, you just have to add the two individual interior angles:
∠RSU = 38° + 28.6° = 66.6°
