To find out when there are 81 players using the function \( f(x) = 3^x \), we need to set the function equal to 81 and solve for \( x \):
\[ 3^x = 81 \]
We know that \( 81 \) can be expressed as a power of \( 3 \):
\[ 81 = 3^4 \]
Now we can rewrite the equation:
\[ 3^x = 3^4 \]
Since the bases are the same, we can set the exponents equal to one another:
\[ x = 4 \]
Thus, there will be 81 players in week \( x = 4 \).