Which i reduced to ...
My work ...
Please help and thanks in advance.
For non-zero a:
In your case with s you've forgot to put the minus sign on the power in the denominator (see my red text)
From your original (and correct) expansion you got
However, in the next step you added the t^12 rather than subtracted it. Since t^12 is in the denominator it needs to have a negative power when multiplied (see the first law above)