The hourly rate is the slope m, and we're given the point (3,75)
Now, using the point-slope form:
[tex]y-75=15(x-3)[/tex]Now, let's put it in the slope-intercept form (clear y):
[tex]\begin{gathered} y-75=15x-45 \\ \rightarrow y=15x+30 \end{gathered}[/tex]For 20 hours of service,
[tex]\begin{gathered} y=15(20)+30 \\ \rightarrow y=330 \end{gathered}[/tex]The service would charge $330
