For a function of two variables, use the ezsurf (or preferably fsurf) function instead:
syms x1 x2
f(x1, x2) = x1^2 + 9*x2^2;
ezsurf(f)
or:
syms x1 x2
f(x1, x2) = x1^2 + 9*x2^2;
fsurf(f)
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