Knowing that the other side of the rectangle is just the perpendicular of the other side then the equation of the line would have a slope of negative reciprocal.
finding the slope of the first line you'll have:
3x - 2y = 12
-2y = -3x + 12
divide the entire equation with -2
y = 3/2x - 6
since the slope of the line is 3/2 then the slope of the perpendicular line is -2/3
since you are not given with point coordinates for the other side then let's have the intercept of the first line.. using the same y-intercept you'll have the equation as:
y = mx + b
y = -2/3x - 6 or
multiplying the whole equation with 3
3y = -2x -2
3y + 2x = -2