If for some x ε R, the frequency distribution of the marks obtained by 20 students in a test is: then the mean of the marks is:

If for some $\displaystyle x \boldsymbol{\epsilon} \mathbf{R}$, the frequency distribution of the marks obtained by $\displaystyle 20$ students in a test is :

Marks	$\displaystyle 2$	$\displaystyle 3$	$\displaystyle 5$	$\displaystyle 7$
Frequency	$\displaystyle (x+1)^2$	$\displaystyle 2 x-5$	$\displaystyle x^2-3 x$	$\displaystyle x$

then the mean of the marks is :