Hi, for part (a) I’ve included a second method that you can use. For part (c), recognise that the basic angle for angle PQS is angle SQR, and since angle PQS is obtuse i.e in the second quadrant, cos PQS will be negative. Therefore cos PQS will be the negative of the cosine of its basic angle i.e. - cos SQR. Leave a comment if you still need help / clarifications!