I am asked to estimate the value of $\displaystyle f_{xy}''(3;4)$ in

http://i43.tinypic.com/2gsh6wn.jpg

I have already found:

$\displaystyle f_x'(3;4) = -5$

$\displaystyle f_y'(3;4) = 2$

How do I find $\displaystyle f_{xy}''(3;4)$ from there?

Thanks a lot!