@Kitonum 

Hi, thanks to your answer. 

I have one more question. If you don't mind, I want you to answer.

> x1=ginseng, x2=perilla, x3=preference;
> f1:=8.044048-0.764286*x1-0.756746*x2+0.034524*x1*x1+0.022222*x2*x1+0.098413*x2*x2;
> plot3d(f1, x1=3..9, x2=1..7, axes=boxed);
> highlightvertex(x1,x2);

             x1 = ginseng, x2 = perilla, x3 = preference

f1 := 8.044048-.764286*x1-.756746*x2+.34524e-1*x1^2+.22222e-1*x2*x1+.98413e-1*x2^2

I want to know the value of the x and y axes for the largest z-axis value.

How can I know the values?