To simplify the expression $6(3x+4) -10x + 9$, follow these steps:

1. Distribute the 6: $6 \times 3x + 6 \times 4 = 18x + 24$

2. Substitute back into the expression: $18x + 24 - 10x + 9$

3. Combine like terms: $(18x - 10x) + (24 + 9) = 8x + 33$

So the simplified expression is $8x + 33$.