From the graph:
[tex]\begin{gathered} (x1,y1)=(1,500) \\ (x2,y2)=(4,2000) \\ m=\frac{y2-y1}{x2-x1}=\frac{2000-500}{4-1}=\frac{1500}{3}=500 \end{gathered}[/tex]Using the point-slope equation:
[tex]\begin{gathered} y-y1=m(x-x1) \\ y-500=500(x-1) \\ y=500x \\ so\colon \\ t=500m \end{gathered}[/tex]