As per the given condition, combination behaves as a plane glass plate, having focal length \(\infty \).

So by using \(\frac{1}{F} = \frac{1}{{{f_1}}} + \frac{1}{{{f_2}}} - \frac{x}{{{f_1}{f_2}}}\)

\( \Rightarrow \) \(\frac{1}{\infty } = \frac{1}{{ + 30}} + \frac{1}{{ - 10}} - \frac{x}{{(+ 30)(- 10)}}\)

\(\Rightarrow x = 20\)\(cm\)