Hi,
In the context of the synthesis of a coordination compound were added 1 mmol of copper (I) bromide with 1 mmol of 2,5-dimethylpyrazine and were stirred in acetonitrile. Here, a solid precipitated from X, which was filtered off and dried.
An elemental analysis gave the following values​​: C: 28.65%, N: 11.12%, H: 3.21%. The content of
Copper and bromine additionally by atomic absorption spectroscopy (AAS) was determined: (Cu: 25.25%, Br: 31.75%).

As ratio formula I've got CuBrC6H8N2.

But what's the molecul formula???

Can somebody help me??

You probably don't need this now, but as far as I can tell, you don't have enough information to determine molecular formula from this without knowing information about the actual reaction, only empirical. You would need something related to the compounds molecular mass to figure out the molecular formula from the empirical one.