The two lines are y=3x+a and y=-x/3+b, where a and b have to be found.
Plug in the point (1,2):
2=3+a so a=-1 and 2=-1/3+b, so b=7/3.
The equations are y=3x-1 and 3y=7-x (another way of writing y=-x/3+7/3). The lines are perpendicular to one another.