This is one of those questions where the devil is in the detail, so more information about what exactly you are trying to achieve will be very helpful in answering your question. So let me list a few of the possibilities:
1) If there is an exact linear relationship between z and (x,y), something like
z=ax+by then finding the values of a and b is a straightforward bit of equation solving.
2) If the relationship between z and (x,y) is linear, but involves the addition of random noise to z, something like
zi=axi+byi+ϵi where
ϵi is normal random noise, then the solution involves minimizing the least squares error, as one does in linear regression.
3) If the relationship between z and (x,y) is non-linear, then it really helps if you know beforehand what the likely form of the relationship is. Solving exact non-linear equations begins to get tricky, but if you've got a random noise component, then you can use extended techniques from linear regression as in (2).
4) If the relationship between z and (x,y) is non-linear, and you don't know beforehand what the form of the relationship should be, then you are into the realms of fitting the solution surface with things called splines.
So, let us know a little bit more about the problem and what you are trying to achieve and we'll do our best to help!
I note also that you wish to avoid using a computer...that will be a challenge for some of the situations above.