The locus of points in a plane that are equally distant to two intersecting lines are lines which are the two bisectors of the given lines. (The distance from a point to a line is found by measuring along the perpendicular to the line.)
Since L1 is vertical and L2 are horizontal, the bisectors will be the two lines which have slopes of 45° (m = 1) and 135° (m = -1).
For this problem, you want the line that passes through the intersection point of the two lines (3, 3) with a slope of -1; defined by the equation y = -x + 6.
You can debate whether or not you want the point (3,3) included in the locus (Is a distance of 0 really a distance?); my preference is to include it.