The rate of change of the line is equal to its slope. To calculate the slope, choose two points on the line and write down their coordinates.
Take the points:
![\begin{gathered} (0,4) \\ (5,6) \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/hnoe9zy4l7f8sejyki46q68ixu63hqu02q.png)
Given two arbitrary points:
![\begin{gathered} (x_1,y_1) \\ (x_2,y_2) \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/kxl4nzdwshwamx8uh0598itwzb86x9vtf4.png)
The slope between them is given by the equation:
![m=\frac{y_2-y_1}{x_2-x_1_{}}](https://img.qammunity.org/2023/formulas/mathematics/college/bc5mz88rq6sxivntwj3pdfog5qhzbibq8g.png)
Substitute y_2=6, y_1=4, x_2=5 and x_1=0 into the formula for the slope to find the rate of change:
![\begin{gathered} m=(6-4)/(5-0) \\ =(2)/(5) \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/jbn6ce5navazt73i3r35hhqw2klatd9n63.png)
Therefore, the rate of change of the vocal pieces that Greta can sing over the weeks that she takes voice lessons, is 2/5.