Simple....
you take any point on that graph and figure out the slope..
you need 2 points though! So keep in mind...pick any two points and find it's slope then find it's y-intercept to find the equation of the line...
Example:
(3,6)
(2,4) (THIS IS AN EXAMPLE RANDOM COORDINATES I MADE!!)
Use y=mx+b....
6=2(3)+b
6=6+b
0=b
y=2x
This is how you figure out the equation of a line using only a graph...
Thus, your answer.