To simplify the expression \( 6(3x−5)+2(3x−5)−8(3x−5) \), we can factor out the common term \( (3x - 5) \):
\[ (3x - 5)(6 + 2 - 8) \]
Now simplify the expression within the parentheses:
\[ 6 + 2 - 8 = 8 - 8 = 0 \]
Thus, we have:
\[ (3x - 5)(0) = 0 \]
So, the simplified expression is:
\[ \boxed{0} \]