8,1 q-3,5 q

It looks like you have 8,1 q - 3,5 q (using comma as decimal separator).

• Factor out q: q*(8,1 - 3,5) = q*(4,6)
• So the result is 4,6 q (in decimal form), which is (23/5) q in fractional form.

If you meant an equation (e.g., = something), please share the full expression.