#### Answer

$a=-3, 3$

#### Work Step by Step

We have:
$f(x)=2x^2+5$ and $g(x)=3x+a$
$(f \circ g)$ crosses the y-axis at 23. This implies that $(f \circ g)(0)=23$
Now, $g(0)=(3)(0)+a =a$
So, $(f\circ g) (0) =f[g(0)]=f(a)=2a^2+5=23$
The above equation can be simplified as:
$2a^2 +5 =23 \\ 2a^2=18 \\ a=\pm 3=-3, 3$