To solve the expression \(( -3 )( 4 - 5 )\), we first simplify the expression inside the parentheses:
\[ 4 - 5 = -1 \]
Next, we substitute this result back into the expression:
\[ ( -3 )( -1 ) \]
Now we multiply the two numbers:
\[ -3 \times -1 = 3 \]
Therefore, the answer to fill in the blank is:
\[ \boxed{3} \]