If you are given all three sides, you could use the cosine law. For finding angle A, the equation would be
a² = b² + c² - 2bccosA
2bccosA = b² + c² - a²
cos A = (b² + c² - a²)/2bc
A = arc cos ((b² + c² - a²)/2bc)
A = arc cos ((28² + 15² - 34²)/2*28*15)
A = arc cos(-0.175)
A = 100.08 degrees
I hope I was able to explain it clearly. Have a good day.
