Can someone explain on how to find this question please?

Answer:
∠ SOU = 138°
Step-by-step explanation:
the opposite angles of a tangent kite are supplementary, sum to 180° , so
∠ SOU + ∠ STU = 180° , that is
∠ SOU + 42° = 180° ( subtract 42° from both sides )
∠ SOU = 138°