To evaluate the expression \( c - d \) where \( c = -20.1 \) and \( d = -23.6 \), we can substitute the values into the expression:
\[ c - d = -20.1 - (-23.6) \]
This simplifies to:
\[ -20.1 + 23.6 \]
Now, we can perform the addition:
\[ -20.1 + 23.6 = 3.5 \]
Therefore, the final answer is:
\[ \boxed{3.5} \]