Optimization help please?

7. A model used for the yield Y of an agricultural crop as a function of the nitrogen level N in the soil(measured in appropriate units) is

$\displaystyle Y=\frac{kN}{1+N^2}$

where K is a positive constant. What nitrogen level gives the best yield?

I thought all optimization problems follow a basic, constraint and then a formula form.

I'm terrible at word problems, so I don't see where there is a constraint in this one?

Please explain these problems to me. they drive me crazy. (Headbang)