Congruence with $x$ in a power

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I don't know how to find $x$ in a situation like this:

$$a^x \equiv b \pmod c$$

I think I'm missing something around little fermat theorem, Could anyone help?

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This is called the discrete log problem and is not trivial, unless you know a primitive root for $c$. For small $c$ you can of course simply keep multiplying by $a$ until you get $b$.