kill(all)$

van=facebook.com/LearningCalculus$

f(x,y,z):=x^2+y^2+z^2-4$
g(x,y,z):=x^3+y^3+z^3-8$
s(x,y,z):=f(x,y,z)-g(x,y,z)$

s(2,0,0);
s(0,2,0);
s(0,0,2);