The slope is -3. Pick any point on the line. For every 1 unit you move to the right, the "next" point on the line is 3 units down.
You can also use the formula for the slope. According to the graph, the line goes through the points, say, (1, -3) and (3, -9). Then the slope is
slope = (-9 - (-3)) / (3 - 1) = (-9 + 3) / (3 - 1) = -6/2 = -3