The slope of two perpendicular lines m1 and m2 are related as m1* m2 = -1.

The slope of the line through (1,3) and ( 2,6) is :

m = (6 - 3)/(2 - 1)

=> m = 3

A line perpendicular to this line has a slope -1/3.

**The slope of the required line is -1/3.**