Hey guys! I've had a couple of troubles trying to show that these propositions are equivalent. Would you mind giving me a hand?

$\displaystyle $\noindent (a) All eigenvalues of A have positive real part \\ \\ (b) For every norm of $M_{n\times n}$. There exist $L>0$ and $\alpha>0$ such such that: $\left \| e^{tA}x \right \|\geq Le^{\alpha}\left \| x \right \| $ \\ For all $t \geq 0 $, $x \in \mathbb{R}^N$ \\ \\(c) There is $a>0$ and a norm $\\ \left \|\ \cdot \ \right \|_\ast$ such that: $\left \| e^{tA}x \right \|\geq Le^{\alpha}\left \| x \right \| $ \\ For all $t \geq 0 $, $x \in \mathbb{R}^N$ $ $

Regards!