Could someone walk me through how to do this?

First of all if you didn't know, all triangles should add up to 180 degrees. We know either A or B is correct. 90 + 60 = 150. 180 - 150 = 30 degrees
30 + 90 + 60 = 180 degrees so it is correct BUT.. 90 + 30 = 120, so 180 - 120 = 60 degrees. 90 + 30 + 60 = 180. So they are both in fact, correct.

If there is only one answer, it will be (B) because its the same triangle just flipped around.