Let the number be \( x \).
When the number is trebled, it becomes \( 3x \).
According to the problem, if we subtract five times the number from the trebled number, we get 3.7. This can be expressed with the equation:
\[ 3x - 5x = 3.7 \]
Simplifying the left side gives:
\[ -2x = 3.7 \]
Now, to find \( x \), we divide both sides by -2:
\[ x = -\frac{3.7}{2} = -1.85 \]
Thus, the number is
\[ \boxed{-1.85} \]