To find the value of the function \( y = \frac{1}{8}x - 13 \) when \( x = -4 \), we can substitute \(-4\) for \(x\) in the equation.
\[ y = \frac{1}{8}(-4) - 13 \]
First, calculate \(\frac{1}{8}(-4)\):
\[ \frac{1}{8}(-4) = -\frac{4}{8} = -\frac{1}{2} \]
Now, substitute this value back into the equation:
\[ y = -\frac{1}{2} - 13 \]
Next, combine the two terms:
\[ y = -\frac{1}{2} - \frac{26}{2} = -\frac{1 + 26}{2} = -\frac{27}{2} \]
Thus, the value of the function when \( x = -4 \) is
\[ y = -\frac{27}{2} \]
or approximately \(-13.5\).