Consider that the graph shows a straight line passing through the points (-3,0), (-2,1), (-1,2), (0,3) etc.
However, only two points are sufficient to obtain the equation of the line.
The equation of the line passing through two points is given by,
[tex]y-y_1=\frac{y_2-y_1}{x_2-x_1}(x-x_1)[/tex]So the equation of the line passing through (-3,0) and (0,3) will be,
[tex]\begin{gathered} y-0=\frac{3-0}{0-(-3)}(x-(-3)) \\ y=\frac{3}{3}(x+3) \\ y=x+3 \\ x-y+3=0 \end{gathered}[/tex]Thus, the required equation is x -