Let a random variable X take values 0,1,2,3 with P(X=0)=P(X=1)=p, P(X=2)=P(X=3) and E ( X^2)=2 E ( X ). Then the value of 8 p -1 is:

Let a random variable $\displaystyle X$ take values $\displaystyle 0,1,2,3$ with $\displaystyle P(X=0)=P(X=1)=p, P(X=2)=P(X=3)$ and $\displaystyle \mathrm{E}\left(\mathrm{X}^2\right)=2 \mathrm{E}(\mathrm{X})$. Then the value of $\displaystyle 8 \mathrm{p}-1$ is :