You can compute the slope given that you have two points [tex](-6,8)[/tex] and [tex](5,3)[/tex].
[tex]m=\frac{3-8}{5-(-6)}=\frac{-5}{11}=-\frac{5}{11}[/tex]
Your function has a form of,
[tex]f(x)=mx+n[/tex]
So we also need to find n. To do that, substitute y and x for some point we already know, I will pick point [tex](5,3)[/tex] then solve for n,
[tex]3=-\frac{5}{11}\cdot5+n[/tex]
[tex]n=3+\frac{5}{11}\cdot5[/tex]
[tex]n=3+\frac{25}{11}[/tex]
[tex]n=\frac{33+25}{11}=\frac{58}{11}[/tex]
So the function is,
[tex]f(x)=-\frac{5}{11}x+\frac{58}{11}[/tex]
Hope this helps :)