Your comments from position

Hello, what do you think about this opening? I play black.

1. d4 d5 2. c4 c6 { D10 Slav Defense } 3. Bf4 dxc4 4. e4 Nf6 5. e5 Nd5 6. Bg3 Qa5+ 7. Nd2 Qb4 8. Rb1 Bf5 9. a3 Qb6 10. Nxc4

I can play c3 but queen can play f3 its noy good. So i think i have to back my queen D1. But i have still a good position with blacks?

You should play 6...b5 after which you are up a pawn with no compensation for him. his 3.Bf4?! is not the best move because 3...dxc4 4.e3 b5 is good for black then, compare (using an engine) with 3.Nf3 dxc4?! 4.e3 b5.

I know for b5, but i tried to push or pick his b2. From what position i make an error? I mean, i beging to enter on difficult position.