perpendicular lines have an negated inverse slope compared to a given line. so we need to find slope of line:
3x + 9y = 7
y = -1 / 3 x + 7
so the opposite of -1/3 is 1/3. flip the fraction to get the inverse and it is 3/1 or 3. 3 is then the slope of perpendicular line. now we see which one will work with it. there are 2 possible
3x+ 4 = y
3x - 14 = y
plug in the point on line and see if it works
3 (6) + 4 = 4 nope
3 (6) -14 = 4 yes
so 3x - 14 = y