work done by A in single day = $\frac{1}{9}$
work done by B in single day = $\frac{1}{12}$
now both works in alternate day, that means in 2 days total work can be done is $\frac{1}{9} + \frac{1}{12}$ _________________(1)
if both works in alternate day and work can be done in $x$ days than work done combinely in single day = $\frac{1}{x}$
so work done in 2 days = $\frac{2}{x}$ _______________________(2)
on comparing both $\frac{1}{9} + \frac{1}{12} = \frac{2}{x}$
$\therefore\text{ x = 10.28 days for combine work}$