numerics – Solving parametric function numerically for positive values

So I have a function f1(x,y) that includes a parameter c. I would like to solve the corresponding f1(x,y)==0 for positive x and y (depending on c), with the further restriction that c>=0, but I do not know how to encode all of this properly.

My try was:

Reduce(f(x,y)==0 && x>0 && y>0 &&c>=0, {x,y})

But it does not seem to work (it is stuck “running”), while playing around a little bit NSolve(f(x,y)==0, {x,y})/. -> c=1, or any positive c have many solutions, among which some for positive x and y. Conversely, also NSolve(f(x,y)==0 &&x>0 && y>0, {x,y})/. -> c=1 is stuck running. Am I doing something wrong?