fsubx is the partial derivative of f with respect to x. So take y as a constant in the f(x,y) expression before differentiating.

Example:

f(x,y)=4x-3y+6

fx(x,y)=4

Similarly, fsuby is the partial derivative with respect to y. Now you will treat x as a constant.

Example:

f(x,y)=3x^2+2xy-7y^2

fy(x,y)=2x-14y

Finally, fxx is the second partial derivative. Just follow the procedure above a second time, with y held constant again.

Hope that helps,

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