Let's look first at your equation y = x/3-1. I must assume that you meant that the slope of this line is (1/3). If so, please enclose that "1/3" in parentheses:
y = (1/3)x - 1, or write the equation as y = x/3 - 1, so that no one wonders whether you meant to combine 3 with -1.
If your equation is indeed y = (1/3) - 1, then the slope of a line perpendicular to this one is the negative reciprocal of (1/3). This is (-3).
Thus, the equation of the perpendicular line is y - 2 = -3(x - 4).
That's in "point-slope" form.