Answer
-1/3
Explanation:
First you want to know the slope-intercept formula which would be y=mx+b.
Now that you know that you want to convert x+3y=3 to slope intercept form. First subtract x on both sides to get 3y=-x+3. Last isolate y by dividing by 3 on both sides of the equation which will leave you with y= -1/3x+1. Since m in the formula y=mx+b stands for slope then the slope of the original equation would be -1/3