To find the value of the function \( y = 18x - 13 \) when \( x = -4 \), we substitute \(-4\) for \(x\):
\[ y = 18(-4) - 13 \]
Now calculate \(18 \times -4\):
\[ y = -72 - 13 \]
Next, subtract \(13\) from \(-72\):
\[ y = -85 \]
So, the value of the function when \( x = -4 \) is \( y = -85 \).