Idon't know how to graph this inequality

y< -3x

-- First, graph the line [ y = -3x ]. 
. . . . That's just the line that goes through the origin and has a slope of  -3.
. . . . Mark the line very lightly on the graph paper, because the line itself is
not part of the final graph.  It's just the edge of the final graph.

-- The line is made of all the points where 'y' IS  -3x .
. . . . You want all the points where 'y' is less than  -3x .
. . . . Those are all the points below the line ... just not the line itself.