For any given equation, how can I figure out what value a given constant has to be in order to give me a certain value of z(x,y)?

Imagine any equation however simple or however complex:

z = @(x,y) x + y + constant;

How can I figure what the value of constant has to be for a given z(x,y)? Obviously, in the above equation I can simply back calculate it like so 'constant = z - x - y'. But I want there to be a way I can easily do this with very complex equations. An example is with the above equation, I would want to know what value 'constant' has to be in order to give me z(4,4) = 24.

Thank you!

[Star Strider]

What didn’t work for you in my Answer to: Find value of a constant given a value of z(x,y) and Matt J’s additional Comment?

What is an actual (or closer to actual) equation you want to estimate? Are you doing more involved optimisation, or curve-fitting?

We can only answer problems we see. We can infer what is actually being asked in some situations if the Question is ambiguous and if we have enough additional information, but asking the same question will only give you essentially equivalent versions of the same Answer.

What Question are you really asking?

[A]

Sorry to make another thread.

With the method described using fzero function, I could only find the value of the constant for when z(x,y) equaled 0. But what if I wanted it to equal something else?