Wicks Theorem and Gaussian Integrals

I am trying to complete A. Zee's book QFT in a Nutshell and at page 15 he mentions Wick's theorem and Wick contractions. (apologies for the huge page-snip).

Why does he mean by connecting the '$2n$ points' and how does the diagram of the $6x$'s prove wick's theorem?

Also why is the final result for just $a^{-3}$ times the number of distinct Wick contractions? How does one derive this? Essentially, what the Wick theorem tells you is that the moments of a multivariate gaussian distribution are determinate by the second moments; for instance, for a $3D$ gaussian in $(x,y,z)$ space, the quantity $$\langle xyzx \rangle$$ can be calculated in terms of $\langle xy\rangle$, $\langle xz \rangle$, $\langle xx\rangle$ and $\langle yz\rangle$. You can indicate these quantities with the contraction symbol that you see in the book; for instance, $\langle xy\rangle = x \, y$ with a bar connecting x and y that I don't know how to put here in MathJax. With the Wick's theorem you find $$\langle xyzx\rangle = \langle xy\rangle \langle zx\rangle +\langle xz\rangle \langle yx\rangle +\langle yz\rangle \langle xx \rangle .$$
In his example, he calculates $\langle x^6 \rangle$; he firs writes it as $\langle xxxxxx \rangle$; the only moment we need to know is $\langle x^2 \rangle$, because using the Wick theorem we can express $\langle x^6 \rangle$ as a sum of terms like $$\langle xx\rangle \langle xx\rangle \langle xx\rangle = (\langle x^2 \rangle)^3 = a^{-3};$$ how many of them? Counting all the possible contractions like he did we get fifteen of such terms, and so $$\langle x^6 \rangle = 15 \, a^{-3}.$$