given y = x-6 where the equation of a line is y = mx + c, i.e. if m = 1. then 1 x x = x.
Now a line that is perpendicular to this perpendicular line must satisfy the formula:
m1.m2 = -1
so, if the slope of our line is 1 (as I have shown) , then the opposite must be -1 , hence m = -1:
y = -1x + c