Answer :
First we determine the slope (m):
[tex]m=\frac{8-1}{1-8}\Rightarrow m=-1[/tex]Now, using one of the points and using the expression:
[tex]y-y_1=m(x-x_1)[/tex]We replace the slope (m) and y1 & x1, that is:
[tex]y-1=-1(x-8)\Rightarrow y-1=-x+8\Rightarrow y=-x+9[/tex]