By definition, the Slope-Intercept Form of the equation of a line is:
[tex]y=mx+b[/tex]Where "m" is the slope and "b" is the y-intercept.
Then, to find the slope and the y-intercept of the line given in the exercise, you only need to solve for "y", in order to write the equation in Slope-Intercept Form:
[tex]\begin{gathered} x+3y=3 \\ 3y=-x+3 \\ \\ y=\frac{-1}{3}x+\frac{3}{3} \\ \\ y=-\frac{1}{3}x+1 \end{gathered}[/tex]Now you can identify that the slope is:
[tex]m=-\frac{1}{3}[/tex]And the y-intercept is:
[tex]b=1[/tex]Therefore, the answer is:
[tex]\begin{gathered} m=-\frac{1}{3} \\ \\ b=1 \end{gathered}[/tex]