You can check your answer for part 2 by using the final answer to verify that length OC is indeed 5sqrt3. Indeed, for the question, there is no easy way to check fast, best is to cover your answers and try doing it quickly again, make sure the logic is every step is correct. Also, for part 1, do note that for area of triangle, most of the time cross product is required. It is possible to do part one with dot product only, but tedious and definitely not worth 4 marks. Do post the question again if you have not learnt cross product yet.