Let's denote the unknown number as \( x \).
According to the problem, we can set up the following equation:
\[ 2x + 3 = 7 \]
To solve for \( x \), we first subtract \( 3 \) from both sides:
\[ 2x = 7 - 3 \] \[ 2x = 4 \]
Next, we divide both sides by \( 2 \):
\[ x = \frac{4}{2} \] \[ x = 2 \]
Therefore, the unknown number is \( \boxed{2} \).