When electric bulbs of same power, but different marked voltage are connected in series across the power line, their brightness will be :

As bulbs are connected in series, the current will be the same.

So, P=$\frac{V^2}{R}$​=$I^{2}R$

As I is constant,

P ∝ R

Now put the formula of resistance of bulb ​​ $R_{bulb }=\frac{{V^2}_{marked}}{P_{marked}}$

Since ​$P_{marked}$ is constant for all the bulbs,

It is proportional to ​${V^2}_{marked}$.

So, P ∝  ${V^2}_{marked}$. 

So, power is proportional to the square of their marked voltage.