How do you find the equation of a straight line on a graph?

Firstly you need to find the gradient and to do this you identify two points on the graph and the gradient is the change in the value of the y values divided by the change in the value of the x values. (y2-y1)/(x2-x1). 

Then to find your 'c' value for the y=mx+c (straight line equation where m=gradient), you either sub in a your x and y values for one point as well as your gradient and find the missing number or you identify where the curve cuts the y axis. At this point you will have your equation of the straight line.