The equation is:
y = -2x + 9
Work and explanation:
We should first find the slope of the graphed line.
Remember that :

rise = how many units we move up/down the y axis
run = how far we move on the x axis
The given slope goes "down 2, over 1" so:
That makes the slope:

If simplified, it gives us -2. So we have figured out the slope, m. Now, to figure out the y intercept, we should look at where the graphed line intercepts the y axis. This happens at (0, 9).
The y intercept is the second number; therefore the y intercept is 9.
Therefore, the equation is y = -2x + 9.