To rewrite the exponential equation \(3^5 = 243\) as a logarithmic equation, you can use the logarithm base 3. The general form for converting from an exponential form \(a^b = c\) to a logarithmic form is:
\[ \log_a(c) = b \]
In this case, \(a = 3\), \(b = 5\), and \(c = 243\). Therefore, the equivalent logarithmic equation is:
\[ \log_3(243) = 5 \]
So the value that Fernando should use is \(5\).