Let's simplify the expression step by step:

First, simplify the terms inside the parentheses:
(6-x) = 6 - x
(-x-3) = -x - 3
(5x-1) = 5x - 1

Now substitute these simplified values back into the expression:
(6 - x) - 2(-x - 3) + 0.9(5x - 1)

Now distribute the coefficients:
6 - x - 2(-x) - 2(-3) + 0.9(5x) - 0.9(1)

Simplify further:
6 - x + 2x + 6 + 4.5x - 0.9

Combine like terms:
(2x + 4.5x) = 6.5x,
(6 + 6) = 12
(-x - 0.9) = -x - 0.9

Combine all terms:
6.5x + 12 - x - 0.9

Combine x terms:
6.5x - x = 5.5x

Finally, combine all terms to get the simplified expression:
5.5x + 12 - 0.9

Therefore, the simplified expression is:
5.5x + 11.1.