When you know the coordinates [tex] (x_0,y_0) [/tex] of a point belonging to a line and its slope [tex] m [/tex], you can write the equation as
[tex] y-y_0=m(x-x_0) [/tex]
Plug your values [tex] (x_0,y_0) = (7,8) [/tex] and [tex] m=-3 [/tex] to get
[tex] y-8=-3(x-7) [/tex]
which you can expand to
[tex] y = -3x+29 [/tex]
