If fx=-1.3 and fy= 2.6 is fxy positive or negative? If fx=-1.3 and fy= 2.6 is fxy positive or negative?
I'm thinking that since fy is positive and greater then fx (who is negative) it will be positive? 
I estimated fx and fy from a contour plot. On the contour plot the f(x,y) is more curves then lines. So maybe fx = -1.3x and fy= 2.6y ?
Thanks for help!

If the partial derivatives $f_x$ and $f_y$ are constant, the function has shape $ax+by+c$, and $f_{xy}$ is identically $0$. 
If the partial derivatives are $-1.3$ and $2.6$ at a particular point, we can say nothing about $f_{xy}$ at that point.
Examples: Let $f(x,y)=-1.3x+2.6y+88xy$. Then $f_x(0,0)=-1.3$, $f_y(0,0)=2.6$, and $f_{xy}(0,0)=88$, positive. 
Using $f(x,y)=-1.3x+2.6y-88xy$, we get the same first partials at $(0,0)$, but $f_{xy}(0,0)=-88$, negative.
Using $f(x,y)=-1.3x+2.6y-x^2y$, we get the same first partials at $(0,0)$, but $f_{xy}(0,0)=0$. 
