To find the value of the function \( y = \frac{1}{8}x - 13 \) when \( x = -4 \), we can substitute \(-4\) into the equation.
\[ y = \frac{1}{8}(-4) - 13 \]
Calculating \(\frac{1}{8}(-4)\):
\[ \frac{1}{8}(-4) = -\frac{4}{8} = -\frac{1}{2} \]
Now substituting this back into the equation:
\[ y = -\frac{1}{2} - 13 \]
To combine the terms, we convert \(-13\) into a fraction:
\[ -13 = -\frac{26}{2} \]
Now we combine:
\[ y = -\frac{1}{2} - \frac{26}{2} = -\frac{1 + 26}{2} = -\frac{27}{2} \]
Calculating \(-\frac{27}{2}\):
\[ -\frac{27}{2} = -13.5 \]
Thus, the value of the function when \( x = -4 \) is \(-13.5\).
So the answer is:
−13.5