Convert Standard Form into slope-intercept form:
3x+by=12
y = (-3/b)x +12/b
Slope of parallel lines is negative reciprocal of this slope (-3/b):
y = (b/3)x + C [note: y-intercept is different,;not 12 ]
and we need point (4,9) to find C [note: we must use y=mx+C because b is already used.]
0 = (b/3)(4) + C
-4b/3 = C
so, the parallel line is:
y = (b/3)x - 4b/3 [ also y = bx/3 - 4b/3 ]