Let $P\left(x\right)$ be a real polynomial of degree $3$ which vanishes at $x=-3$. Let $P\left(x\right)$ have local minima at $x=1$, local maxima at $x=-1$ and ${\int }_{-1}^{1}P\left(x\right)dx=18$, then the sum of all the coefficients of the polynomial $P\left(x\right)$ is equal to $?$