To make things easier, put like terms together (so according to their unknown).
9a+2b-8a-3b ≡ 9a-8a+2b-3b
Now you can easily see what to subtract, and what to add.
eg: 9a-8a = 1a (or 'a')
2b-3b = -1b (or '-b')
Which means the simplified format for 9a+2b-8a-3b is a-b.
ans = 16a+13b