We have tjhe general rule for dilations:
[tex]D_k(x,y)=(kx,ky)[/tex]
where k is the scale factor.
In this case, we can take the points (10,0) and the dilated point (8,0) to find the scale factor like this:
[tex]\begin{gathered} D_k(10,0)=(10k,0k)=(8,0) \\ \Rightarrow10k=8 \\ \Rightarrow k=\frac{8}{10}=\frac{4}{5} \\ k\text{=}\frac{4}{5} \end{gathered}[/tex]
therefore, the scale factor is k=4/5
