You will find the required series for sin x and cos x here: Taylor series - Wikipedia, the free encyclopedia
Now apply the advice given in post #4.
Does your answer satisfy
1 + a - b = 0 .... (1)
3a + b = -6 .... (2)
You should know how to solve simultaneous equations at the level you're apparently studying at. Personally, I'd just add equations (1) and (2) and solve for . Then I'd substitute my value for into (1) and solve for .