Understanding Power Series Multiplication Step Working on Spivak's Calculus problems, I searched online, trying to understand the solution provided for Problem 4a of Chapter 2. I found the question I needed:
Spivak's Calculus - Exercise 4.a of 2nd chapter.
However, the answer provided there started with the following equation, and then went on from it to explain other things, which I could understand. But this part, which lies at the foundation of the argument, I don't understand.
Could I get an explanation for why it is that:
$$
\begin{align}
\left(\sum_{k=0}^\infty a_kx^k\right)\left(\sum_{k=0}^\infty b_kx^k\right)
&=\sum_{k=0}^\infty\left(\sum_{j=0}^k a_j\color{#C00000}{x^j}b_{k-j}\color{#C00000}{x^{k-j}}\right)
\end{align}
$$
Keep in mind I'm a beginner, working on an introductory Calculus book as my first exposure to the subject. I'd appreciate both intuitive and rigorous answers.

Write the product out longhand:
$$\left(a_0+a_1x+a_2x^2+a_3x^3+\ldots\right)\left(b_0+b_1x+b_2x^2+b_3x^3+\ldots\right)\;.$$
This is the sum of all possible products of the form $(a_kx^k)(b_\ell x^\ell)=a_kb_\ell x^{k+\ell}$. The $x^n$ term in the product will therefore be the sum of all of these term $a_kb_\ell x^{k+\ell}$ for which $k+\ell=n$. Let’s look specifically at $n=3$, just to get a clearer picture of what’s going on. If $k+\ell=3$, where $k$ and $\ell$ are exponents in the two factors, then clearly $\langle k,\ell\rangle$ must be one of the pairs $\langle 0,3\rangle,\langle 1,2\rangle,\langle 2,1\rangle$, or $\langle 3,0\rangle$. Thus, the $x^3$ term in the product is
$$(a_0x^0)(b_3x^3)+(a_1x^1)(b_2x^2)+(a_2x^2)(b_1x^1)+(a_3x^3)(b_0x^0)=\sum_{k=0}^3(a_kx^k)(b_{3-k}x^{3-k})\;,$$
or
$$\left(a_0b_3+a_1b_2+a_2b_1+a_3b_0\right)x^3=\left(\sum_{k=0}^3a_kb_{3-k}\right)x^3\;.$$
If you generalize this to an arbitrary power $n$ of $x$, you get 
$$(a_0x^0)(b_nx^n)+(a_1x^1)(b_{n-1}x^{n-1})+\ldots+(a_{n-1}x^{n-1})(b_1x^1)+(a_nx^n)(b_0x^0)=\sum_{k=0}^n(a_kx^k)(b_{n-k}x^{n-k})\;,$$
or
$$(a_0b_n+a_1b_{n-1}+\ldots+a_{n-1}b_1+a_nb_0)x^n=\left(\sum_{k=0}^na_kb_{n-k}\right)x^n\;.$$
In other words, the coefficient of $x^n$ in the product series is
$$\sum_{k=0}^na_kb_{n-k}\;.$$
