A contour map for a function $\displaystyle {f}$ is shown.

[Graphs generated by this script: initPicture(-5,5,-1,9);axes(1,1,1,1,1);stroke='blue';circle([0,1],1.1);circle([0,2],2.2);circle([0,3],3.3);circle([0,4],4.4);circle([0,5],5.5);fontfill='blue';text([1,2],'10');text([1.8,3.7],'20');text([2,6],'30');text([3,8],'40');text([4.5,1],'50');]

Use the contour map to estimate the value of $\displaystyle {{f}_{{x}}{\left(-{2},{1}\right)}}$

$\displaystyle {{f}_{{x}}{\left(-{2},{1}\right)}}\stackrel{\sim}{=}$