You have to decide where the plotted line segment should start and end.
Here's one way you might do that, which is to plot the line between two points whose x-coordinates are a certain amount away from x1:
x1 = 3;
y1 = 4;
% slope = 2:
m = 2;
% plot the point (x1,y1):
plot(x1,y1,'ko','MarkerFaceColor','k');
hold on
% plot a line segment with x going from x1-1 to x1+1.
% note that if x changes by 1, then y changes by m
% (that's the definition of the slope of a line):
plot(x1+[-1 1],y1+m*([-1 1]),'b','LineWidth',2)
% another line segment going from x1-4 to x1+2,
% with the same slope:
plot(x1+[-4 2],y1+m*([-4 2]),'--r')
axis equal
