What are you given are two sides and the angle in between those sides. Because it's side-angle-side as given left to right, it's called an SAS triangle. When given this combo, we use the L aw of Cosines.
b² = a² + c² - (2ac * cos (B) by the formula for Law of Cosines - remember it as side squared plus side squared minus double the sides and the cosine of the included angle
b² = 65² + 71² - (2 * 71 * 65 * cos (123°)) where we put in our two sides and angle
b² = 4225 + 5041 - (9230 * cos 123°)
b² = 9266 - (9230 * -0.54463903501) by evaluating the cosine of 123 degrees
b² = 9266 - (-5027.01829319)
b² = 9266 + 5027.01829319 by distributing the negative sign outside
b² = 14293.0182932
b = 119.553411
Rounded to one place, b is 119.6 and choice "A" is the right measurement.
