Perform the addition and subtraction below by first rewriting the fractions with the least common denominator $\displaystyle{18}{a}{b}\usepackage{cmbright}\Huge\$ . Then supply the missing coefficient.

$\displaystyle\frac{{1}}{{{6}{a}}}+\frac{{1}}{{{9}{b}}}-\frac{{1}}{{{18}{a}{b}}}=\frac{{{3}{b}+?{a}-{1}}}{{{18}{a}{b}}}\usepackage{cmbright}\Huge\$

The question mark stands for the number _____.