Since the line is parallell to the x-axis we know the slope is zero: [tex]m=0[/tex]
Using the point-slope formula we can substitute know values:
[tex]y=mx+b[/tex]
[tex]4=0(3)+b[/tex]
[tex]b=4[/tex]
[tex]y=0x+4[/tex]
[tex]y=4[/tex]
You could also jsut do it by inspection: since it passes through (3,4) and is horizontal it must cross at (0,4) i.e. all y=4.
