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An oblique triangle has no right angle, so the law of cosines does the work: cos A = (b² + c² − a²)/(2bc). In SAS mode the missing side is found first from a² = b² + c² − 2bc·cos A, then the angles follow.
Law of cosines
a² = b² + c² − 2bc·cos A, cos A = (b² + c² − a²) / (2bc)
Heron's area
Area = √[s(s−a)(s−b)(s−c)]
cos A = (49 + 81 − 25)/126 = 105/126, giving A ≈ 33.5573°, B ≈ 50.7035°, C ≈ 95.7393°.
Arcsine cannot distinguish an angle from its supplement, so the law of cosines is safer when the triangle may be obtuse.