Rationalize the denominator and simplify. Then supply the missing number.

$\displaystyle\frac{{\sqrt{{{3}}}+\sqrt{{{7}}}}}{{\sqrt{{{3}}}+\sqrt{{{2}}}}}={3}-\sqrt{{6}}+\sqrt{{21}}-\sqrt{?}\usepackage{cmbright}\Huge\$

The question mark stands for the number _____.