Question

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If 81 came out, what was dropped in?
I have rewritten the problem to $3^x=81$ but do not know how to solve it.

Answer 1

It is good equation. Look, that:
81 = 3 x 3 x 3 x 3 = 3^4.
So you have to solve:
3^x = 3^4 
exponents have to be the same, so
x=4
That's mean that A(4) = 81

Answer 2

$A(x)=3^x=81\\ 3^x=81\\ 3^x=3^4\\ x=4$