Personally I would solve it by flipping the fraction and making the power +3

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)

For t:

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)