Hi ! i have this exercise , i need help

For a fixed element a $\displaystyle \in{} \mathbb{R}$ we have $\displaystyle C(a)=r \in{} R\ /\ ra=ar$ . Prove that $\displaystyle C(a)$ is a subring of R containing a . Prove that the center of R is the intersection of the subrings $\displaystyle C (a)$ for all s $\displaystyle \in{}$ R