[
  "In a parallelogram, adjacent angles sum to 180°.",
  "Let the two angles be 3u and 7u. Then 3u + 7u = 10u = 180°, so u = 18°.",
  "∠STU is opposite ∠SVU (which is 3u). So ∠STU = 3 × 18° = 54°."
]