The equation of the line that is given, 3x - y = 18, can be written as y = 3x - 18. This is the slope, intercept form. The slope of the given line is 3. As the product of slope of two perpendicular lines is -1, the slope of the line to be determined is -1/3.
This line has to pass through the point (2, 4), the equation of the line is (y - 4)/(x - 2) = -1/3
=> 3(y - 4) = -(x - 2)
=> 3y - 12 = 2 -x
=> x + 3y - 14 = 0
The equation of the required line is x + 3y - 14 = 0