As two pendulums begin two swings simultaneously, then
$n_1{T}_1=n_2{T}_2$
Where, $n_1$ and $n_2$ are the number of oscillations of first and second pendulum respectively and $T_1$ and $T_2$ be their respective time periods.
The time period of simple pendulum is given by
$T=2\pi \sqrt{\frac{l}{g}}$
where, l =length of pendulum
and g = acceleration due to gravity
$\Rightarrow T^2\propto 1$ …. (ii)
So, from Eqs. (i) And (ii), we get
$\frac{l_1}{l_2}=\frac{T_1^2}{T_2^2}=\frac{n_2^2}{n_1^2}$
Here, $n_1=9,n_2=7$
$\Rightarrow \frac{l_1}{l_2}=\frac{{\left(7\right)}^2}{{\left(9\right)}^2}\frac{49}{81}$
Hence, the ratio of pendulum lengths
$l_1:l_2=49:81$