\(l:y=m_1x+b_1\ and\ k:y=m_2x+b_2\\\\l\ ||\ k\iff m_1=m_2\\------------------------\\l:4x-7y=5\to-7y=-4x+5\to y=\frac{4}{7}x-\frac{5}{7}\\\\k:9x+my=3\to my=-9x+3\to y=-\frac{9}{m}x+\frac{3}{m}\\\\l\ ||\ k\iff-\frac{9}{m}=\frac{4}{7}\\\\cross\ multiply:\\\\4m=-9\cdot7\\\\4m=-63\ \ \ \ /:4\\\\m=-15.75\leftarrow answer\)