Here's a graph that might help explain what geno is describing.......
https://www.desmos.com/calculator/rgris7izxp
Note that the diagonal lines, y= x and y =-x + 6, create a set of points ( a "locus' ) that are all equi-distant from the lines x = 3 and y = 3.
But the one that we're interested in is the one with the negative slope, y = -x + 6, which goes through the point (3,3).
{I agree with geno, (3,3) should be included!! }
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