How would you write x+3y=3 in y=mx+b format

X+3y=3
First add subtract the x
3y=-x+3
Then divide 3
Y=-1/3x+1

Your first step to putting x+3y=3 into slope-intercept form is to move x to the other side using subtraction.
 x+3y= 3
-x-x
3y=-x+3

And the final step is to divide both sides by 3 so that you can isolate y and achieve slope-intercept form.
3y=-x+3
/3    /3
y=-1/3x+1

So, x+3y=3 in slope-intercept form would be y=-1/3x+1, where the slope (rise/run) is -1/3 and the line intersects the y-axis at point (0,1).
Remember, in y=mx+b, m represents slope and b represents the y-intercept of the equation's line when graphed.