When using the elimination or substitution method to solve a system of linear equations, how can you tell if a system of linear equations has no solution? what is the relationship between the graphs of the equations?

You can tell if there is no solution if you end up with a false statement after performing elimination. For example: 
-x - y = 4
 x + y = 6
  Add the equations together
 -x - y = 4
 x + y = 6
 0 + 0 = 10
 0 = 10. 
 The line "0 = 10" is false, therefore there is no solution. 
 If the equations were to be graphed, the resulting lines wouldn't intersect each other. So they would not share any points in common.