For part a) you need to use the summation formula for the sine function. You have to recognize that sin(270) = -Sin(90) = -1 and cos(270) = cos(90) = 0.
Then you have
This should not be surprising since 270-90 = 180 and we know that the sine function is zero there. It is a very similar situation in b) and c) and you should be able to do it yourself. Try it out and if you can't do it, show your work and ask for help!