Let $R$ be a commutative ring and $a_1, \dots, a_n$ elements of $R$. Prove that $\frac{R[x_1, \dots, x_n]}{(x_1-a_1, \dots, x_n-a_n)} \cong R.$
$\ker \phi = (x_1-a_1, ..., x_n-a_n)$ for a ring homomorphism $\phi: R[x_1, ..., x_n] \to R$