We give an extremely detailed argument. Sorry about the length!
Suppose first that $p\equiv 1\pmod{6}$. So $p=6k+1$ for some $k$.
We will use a primitive root argument. Let $g$ be a primitive root of $p$. In group-theoretic terms, let $g$ be a generator of the multiplicative group modulo $p$.
Then $g$ has order $p-1$ modulo $p$. Note that $g^{p-1}=(g^{2k})^3\equiv 1\pmod{p}$ by Fermat's Theorem. Also, $(g^{4k})^3\equiv 1\pmod{p}$. Neither $g^{2k}$ nor $g^{4k}$ is congruent to $1$ modulo $p$, since the order of $g$ is $6k$. And they are not congruent to each other.
Thus the congruence $z^3\equiv 1\pmod{p}$ has at least $3$ (and therefore exactly $3$ solutions, namely $1$, $g^{2k}$, and $g^{4k}$.
So if $b^3\equiv a\pmod{p}$, then we also have $(g^{2k}b)^3\equiv a\pmod{p}$ and $(g^{4k}b)^3\equiv a\pmod{p}$. It follows that the congruence $x^3\equiv a \pmod{p}$ has $3$ solutions if it has a solution.
Now suppose that $p\equiv 5\pmod{6}$. Let $p=6k+5$. Let $x$ and $y$ be non-zero integers, and suppose that $x^3\equiv y^3\pmod{p}$. This is the case if and only if $(xy^{-1})^3\equiv 1\pmod{p}$. We show that this forces $x\equiv y\pmod{p}$.
To do this, it is enough to show that the congruence $z^3\equiv 1\pmod{p}$ has only the obvious solution $z\equiv 1\pmod{p}$.
Again let $g$ be a primitive root of $p$. Any $z$ is congruent to $g^m$ for some $m$ with $1\le m\le p-1$. Ig $z^3\equiv 1\pmod{p}$ then $g^{3m}\equiv 1\pmod{p}$. It follows that $3m$ divides the order of $g$, that is, $3m$ divides $6k+4$. Since $3$ and $6k+4$ are relatively prime, it follows that $m$ divides $6k+4=p-1$. Thus $g^m\equiv 1\pmod p$, and therefore $z\equiv 1\pmod{p}$.
Now consider the mapping $\psi$ that takes any number $x$ between $1$ and $p-1$ into the remainder when $x^3$ is divided by $p$. By the calculation above, the function is one to one (injective). A one to one function from a finite set to itself must be onto (surjective). It follows that every number between $1$ and $p-1$ is $\psi(x)$ for some $x$. This says that every number between $1$ and $p-1$ is congruent to $1$ modulo $p$. That is what we wanted to prove.
We know that $2^{28} \equiv 1 \bmod 29$, i.e. $16^7 \equiv 1 \bmod 29$. However we can also conclude from this that:
$16^{14},16^{21},16^{28},16^{35},16^{42},16^{49} \equiv 1 \bmod 29$.
So $16^2, 16^3, 16^4, 16^5, 16^6, 16^7$ are the other $6$ solutions.
Alternatively: we know every $x\in\mathbb{Z}/29\mathbb{Z}$ can be written as $x=2^k$ for some $k$.
To satisfy $x^7 \equiv 1 \bmod 29$ we require $2^{7k} \equiv 1 \bmod 29$. But since $2$ is a primitive root mod $29$ this means that $7k \equiv 0 \bmod 28$, i.e. $k \equiv 0 \bmod 4$ so we may take $k=0,4,8,12,16,20,24$ (after this the powers of $2$ repeat).
Best Answer
$$\begin{array}{rcl} x^2 &\equiv& 1 \pmod{15} \\ (x-1)(x+1) &\equiv& 0 \pmod{15} \end{array}$$
So, $x=1$, $x=-1$, $x=4$, or $x=-4$.