Answer
$\frac{3}{4}$.
Work Step by Step
If a line's equation is $y=mx+b$, then $m$ is its slope. Two lines are parallel if their slopes are equal.
$3x-4y=12\\3x-12=4y\\y=\frac{3}{4}x-3$.
Hence we can see that the slope of a parallel line is $\frac{3}{4}$.
