Explanation:
2x + 1 + x - 13 = 180° because these angles are supplementary
3x - 12 = 180°
3x = 180 + 12
3x = 192 divide both sides by 3
x = 64
2x + 1 ➡ 2 × 64 + 1 = 129°
x - 13 ➡ 64 - 13 = 51°
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