How to treat a y when deriving a function? My econ class gave these questions.
Find derivatives for the following functions:
$$g(z) = z + y + 15$$
$$f(z) = yz + 2y$$
I'm not sure how I should treat the $y$. Is it a function or a variable?
If it is a variable $g'(z)=2$ and $f'(z)=y+2$ but as a function $g'(z)=1+y'$ and $f'(z)=zy'+y+2y'$.

As a side note, the verb form of "derivative" is "differentiate" not "derive". You take some formulas and theorems and "derive" another formula or theorem. You "differentiate" a function to find its derivative. 
To answer your question, it depends on what you're taking the derivative with respect to. Since you write $g(z)$, it is safe to assume that $y$ is a constant and may be treated as such. Thus you treat $y$ like it was any number.
