What is the equation of a line that passes through the point (4,2) and is perpendicular to the line whose equation is y= x/3-1 ?

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.

Understanding what? lol