Interval of Concavity and Inflection point

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I was wondering if there is any way of finding an interval of concavity for a function if the second derivative is undefined for $x$. The function we got was $f(x)=e^x + e^{-3x}$. I realised halfway through trying to find $f''(x)$ that it is undefined.

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$f''(x)=e^x+9e^{-3x}$. The equation $f''(x)=0$ has no real solutions for $x$, thus $f$ never changes concavity. $f''(0)=1+9=10 > 0$ thus $f$ is concave up across all of $\mathbb{R}$.