Use the two-points formula for the slope of a line.
We know that the points (2,-2) and (9,3) are on the line. If (a,b),(c,d) are points on a line then the slope m is defined by the equation . In this case, a=2,b=-2,c=9 and d=3 then the slope is .
Now the problem is that this expansion does not match the given one. As a matter of fact, since is odd, there is no cosine series. So I'm starting to think this question is missing some initial details.
One possibility is that you're actually supposed to use the even extension of , which is to say we're actually considering the function
and enforcing a period of . Now, you should find that
The value of the sum can then be verified by choosing , which gives