********** Note: With parallel lines, the slope has to be the same. ***************
Looking at this equation, it may be hard to tell what the slope is at first, but if you put the equation into slope intercept form it will be much easier to tell. You can do this my simply subtracting 3x from both sides and dividing by 4 to get y alone.
This is what the work should look like:
3x + 4y = 12
4y = -3x + 12
y = -3/4x + 3
This is the original equation rewritten to make it easier to see the slope. Now, knowing that slope of prallel lines has to be the same, look at the slope and choose the answer that is the same exact slope as the original line equation.
Your answer should be: -3/4