Answer
$b^6$
Work Step by Step
The $r+1$ th term in the binomial expansion of $(x+y)^n$ is:
$\binom{n}{r}x^{n-r}y^r$
Here $x=a$ $y=-\sqrt{b}$, $n=12$:
The 13th term.
$r+1=13$
$r=12$
$\binom{12}{12}a^{12-12}(-\sqrt{b})^12=b^6$
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