If by "coincide", in other words this is also known as intersect, it means that the solution to the system is that intersection point. For instance, if the intersection point of the 2 lines is (5,7) on the graph, it means that in both equations, x = 5 and y = 7. If you mean that the 2 equations result in the same line, it means that the 2 equations were really equal, but that one could be basic to be identical with the other. For example, if you have 2x + 2y = 8 and 4x + 4y = 16, they will give you the same line, and you can see that if you divide that second equation by 2 all the way crossways, it merely shows the first equation.
A positive number divided by a negative number gives you a negative number for example 12 is positive but the three is negative so our answer has to be negative
