The easiest way to do this is by graphing the equation. I'll do it:
We see that the y intercept is 11.
Formula for slope-intercept: y = mx + b
mx = slope
b = y-intercept
Now we have y = mx + 11. What's mx? We can find that out from the graph.
Let's find two points: (0,11) (4,3)
Using (y2 - y1)/x2 - x1): We see that the slope is -2.
Now we finish the slope-intercept form and plug it all in:
y = -2x + 11