Let $G$ be a strongly regular graph with $n$ vertices and parameters $k,a,c.$ Show that

$($a$)\frac{?}{\text{b}\text{a}\text{r}}(G)$ is strongly regular with parameters $\frac{?}{\text{b}\text{a}\text{r}}(k)=n-k-1,=n-2-2k+c,$ and $\frac{?}{\text{b}\text{a}\text{r}}(c)=n-2k+a.$

$($b$)$ Assume $n=21,a=3,c=6.$ Find the spectrum of $G.$