Answer
$(0,1) \cup (1,\infty)$
Work Step by Step
The function $y=b^x$ is concave up when $y''$ is positive. We see that:
$y''=b^x(\ln b)^2\ge0$
Note that for $b=1$, the graph is not concave up $(y''=0)$ because the graph is a straight line ($y=1^x=1$). Thus, $y$ is concave up for any value of $b$ except $1$:
$(0,1) \cup (1,\infty)$
