The slope of the line m can be computed using the equation

We have two points presented in the graph, which are (0, 15) and (3, 0). Substitute the values of x's and y's on the equation above and compute. We have

For the y-intercept of the line given, this is where the value of x is equal to zero. As can be seen in the plot, at x = 0, we have y = 15. This means that the y-intercept is equal to 15.
For the x-intercept of the line given, this is where the value of y is equal to zero. As can be seen in the plot, at y = 0, we have x = 3. This means that the x-intercept is equal to 3.
For the equation of the line, we have the general formula

where b is the y-intercept of the plot. Just substitute the value of m and b on the equation above to write the equation of the line. We have
