s[t_] := 500 - 5t^2;

Plot[s[t], {t,0,10}]

slope[t1_,t2_] :=  (s[t2]-s[t1])/(t2-t1)

Manipulate[
Plot[{s[t], slope[t1,t2]*(t-t1)+s[t1]}, {t,0,10},
PlotLabel ->"slope = " <> ToString[slope[t1,t2]]],
{{t1,0},0,10},{{t2,10},0,10},ContinuousAction->False]