To find the value of \( f(x) \) at \( x = -5 \) using the function you provided \( f(x) = 5x \), we can substitute \( -5 \) in for \( x \):
\[ f(-5) = 5(-5) \]
Calculating this gives:
\[ f(-5) = -25 \]
So, the value of \( f(x) \) for \( x = -5 \) is \(-25\).