Let f(x; y; z) = (x^3+3xy^2-3xyz-z^4)/(x^2 + z^2)

Calculate

a. @f/@x

b. @f/@y

c. @f/@z

I suppose that the solution for the partial derivative of the function to x is too simple as I get: 3(x^2+y^2-yz)/2x

Am I right when I have to use the quotient rule? as it is f´g - g´f / g^2......But what is then g only x^2 or x^2+z^2 and is f (x^3+3xy^2-3xyz-z^4) or just (x^3+3xy^2-3xyz) (as the unused variables are held constant?

so is this approach correct:

(3x^2+3y^2-3yz)*(x^2) - 2x(x^3+3xy^2-3xyz) / x^4

thank you in advance....