The slope m is given by:
[tex]\begin{gathered} m=\frac{y2-y1}{x2-x1} \\ \text{Where:} \\ (x1,y1)=(v,-3) \\ (x2,y2)=(8,5) \\ m=-8 \\ so\colon \\ -8=\frac{5-(-3)}{8-v} \\ \text{solve for v:} \\ -8(8-v)=5+3 \\ -64+8v=8 \\ 8v=72 \\ v=\frac{72}{8} \\ v=9 \end{gathered}[/tex]