8,1 q-3,5 q

It looks like you want to combine like terms in 8,1 q − 3,5 q (using comma as decimal).

• Convert: 8.1q − 3.5q
• Subtract coefficients: (8.1 − 3.5)q = 4.6q

So the simplified form is 4.6 q (which is also 23/5 q). If you meant something else (an equation, or a different expression), please share more context.