The slope of a line between #(1,2)# and #(-1,-4)# is

#color(white)("XXX")color(green)(m)=(Deltay)/(Deltax)=(2-(-4))/(1-(-1))=6/2color(green)(=3)#

The general form of a slope-point equation is

#color(white)("XXX")y-color(blue)(b)=color(green)(m)(x-color(red)(a))#

for a line with slope #color(green)(m)# through a point #(color(red)(a),color(blue)(b))#

We could use either #(color(red)(1),color(blue)(2))# or #(color(red)(-1),color(blue)(-4))# for our point #(color(red)(a),color(blue)(b))#

Using #(color(red)(1),color(blue)(2))# the slope-point equation would be:

#color(white)("XXX")y-color(blue)(2)=color(green)(3)(x-color(red)(1))#