OK.........
1) The answer simplified would be......
-t^2 +9s - 4t
2) The answer with the values in place would be
13
3) I prefer the way of which you plug in all the values and simplify. The reason I like that better is when all of the values are plugged in, is all you have to do is simplify instead of removing and moving the numbers around.
H0P3 It H3LPS :)