To simplify this expression, we must combine like terms. This means that we have to add or subtract the coefficients of the same variables, meaning we can only add variable a's together and variable b's together, they must stay separate.
3a + 2b - 8a + b
The first thing that we should do is to rearrange this expression so that the terms with the same variables are next to one another.
3a - 8a + 2b + b
Next, we can combine these terms by adding/subtracting their coefficients.
-5a + 3b
We can also write this as:
3b - 5a
Therefore, your answer is 3b - 5a.
Hope this helps!