How do you find the average rate and instantaneous rate given a kinetics plot?
This is more of a question of calculus than chemistry, but in a first order reaction 
$\ce{A -&gt; B}$, the average rate is:

$$\frac{\Delta A}{\Delta t} = \frac{A_2 - A_1}{t_2 - t_1}$$



 Graciously borrowed from here

So, the average rate is the slope of a secant line between the two points, 

$$ \frac {\Delta A}{\Delta t} = \frac{1 - 0.5}{400 - 200}$$

The instantaneous rate is the derivative of the plot:

$$\frac{dA(t)}{dt}$$ or $$\lim_{t\rightarrow 0} \frac{\Delta A}{\Delta t}$$

For the above first order rate law, the equation is in the form:

$$ A(t) = e^{-kt + ln(A_0)} $$ so the derivative is simply

$$ \frac{dA(t)}{dt} = -ke^{-kt + ln(A_0)} $$