-5. 1a-4b-4.9a+ b

To simplify the expression -5.1a - 4b - 4.9a + b, first combine like terms.

Group the 'a' terms together: (-5.1a - 4.9a) + (-4b + b)

Now, simplify each group: -10a + (-3b)

So the simplified expression is -10a - 3b.