In the slope intercept form (y = mx+b), the variable y is just y, so we need to basically solve for y. First and only, we subtract x from both sides to get:
y = -x+6
For extra information, the variable m is the slope, so it is what x is being multiplied by. On a graph, the slope determines how steep the line is. b is the y intercept, or whatever you are adding onto mx. It is the location that the line on a graph crosses the y axis. With the equation we have here, the slope, or m, is -1 and the y intercept, or b, is positive 6.