The coordinates of point C are (0, 0), while the coordinates of point D are (7, 12).
Applying the formula for the slope, we get
[tex]m = \dfrac{12-0}{7-0}=\dfrac{12}{7}[/tex]
The equation in the slope-intercept form is given by
[tex]y-y_0=m(x-x_0) \implies y-0=\dfrac{12}{7}(x-0) \iff y=\dfrac{12}{7}x[/tex]
The domain and range are, respectively, the portion of the x and y axes covered by the part of the graph you're interested in.
So, the x axis is spanned from 0 to 7 (which is the domain), and the y axis is spanned from 0 to 12 (which is the range)
