I am struggling with the following question -does anyone have any ideas on how to approach it?

Y has the probability density function defined by: f(y) = kθ^k / y^k+1, y >= θ

and f(y) = 0 for y< θ, where θ>0 and k>0

Question: Why is k restricted to being greater than 2 in the definition of the pdf?

Thanks