12/17÷15-3/34×3/15
=3×4/17×1/15 - 3/34×3/15
= 4/17×3/3 - 1.5/17×3/15
= （4/17-1.5/17）×3/15
= 2.5/17×1/5
= 0.5/17
= 1/34