Okay so every triangle will have 3 angles that add to 180 degrees. You are given that A=65 which means that B + C will have to add to 115. You are given that B= (3x-10) and C=2x, plug this into B+C=115. So
3x-10+2x=115, isolate x, 5x=125, divide both sides by 5 and you get that x=25. This makes B=65, and C=50. Just to check, if you add these together you get 180 degrees.