(s^3−2s−9)+(2s^2−6s^3+s) = s^4-6s^3+2s^2-2s

What Am I getting Wrong Here?

Write two subtraction equations that are equivalent to 5 + 12 = 17.

How many credit hours will a student have to take for the two tuition costs to be equal? round the answer to the nearest tenth of an hour.

How many different three-digit numbers can be formed using the digits 1 comma 2 comma 9 comma 6 comma 4 comma 3 comma and 8 without repetition? for example, 664 is not allowed.