how do you know if two straight lines on a graph are parallel or perpendicular?

On a straight line graph, the lines are formed by an equation in the form y=mx + c. The m value in this equation represents the gradient, and the c value is the y-intersect.If two lines are parallel, this means that they have the same gradient, and so if the m values are the same, the lines will be parallel. For example, y=3x+2 and y=3x-1 are parallel. They are the same line, just translated down three on the y-axis.For two lines to be perpendicular their gradients (the m values) must multiply to make -1. For example the product of 4 and -1/4 is -1, and so the two lines y=4x-6 and y=3-1/4x will be perpendicular.