Perpendicular lines have opposite and reciprocal slopes. So first we have to find the slope of the original line. We can do this by solving the equation for y.
5x - 9y = 1
-9y = -5x + 1
y = 5/9x - 1/9
To do the opposite, take the original slope (5/9) and change the sign (-5/9).
To do the reciprocal, take the slope we changed (-5/9) and flip it as a fraction (-9/5).
