#### Answer

$x^{\frac{3}{8}}$

#### Work Step by Step

Step 1: Since both the radicands multiply together, their indexes add to each other.
Step 2: Denoting the missing exponent by $a$,
$\frac{4}{8}=\frac{1}{8}+a$
Step 3: Therefore, $a$ = $\frac{4}{8}-\frac{1}{8}$ = $\frac{3}{8}$
Step 4: The missing expression is thus $x^{\frac{3}{8}}$