question about positive concave functions.

question about positive concave functions.

I am quoting a line from a text:
The Laplace exponent $\Phi$ is concave and non-negative, the inequality
$\Phi(\lambda)\leq k\Phi(\lambda/k)$ for all l$\lambda>0$ and $k>1$
follows.
Why does being concave positive means $\Phi(\lambda)\leq
k\Phi(\lambda/k)$? Can someone please explain? thanks.
Note the $\Phi$ function is defined from $[0,\infty)$ to $\mathbb{R}$. It
is the log of the Laplace transform