To find the value of \( 6.9 - 20 \), you compute it as follows:
\[ 6.9 - 20 = 6.9 - 20.0 = -13.1 \]
None of the provided responses (5.9, 6.9, 7.9, 4.9) match this answer. Therefore, it seems there might be a mistake in the responses given. The correct answer is \(-13.1\).