Two angles are supplementary if the sum of their measures is 180 degrees. Given that \( m\angle QRS = 33° \) and \( m\angle RST = (2x + 21)° \), we can write the equation:
\[ m\angle QRS + m\angle RST = 180° \]
Substituting the given values into the equation:
\[ 33° + (2x + 21)° = 180° \]
Now we can combine like terms:
\[ 33 + 21 + 2x = 180 \]
This simplifies to:
\[ 54 + 2x = 180 \]
To isolate \( 2x \), we subtract 54 from both sides:
\[ 2x = 180 - 54 \] \[ 2x = 126 \]
Thus, the equation that corresponds to the problem is:
\[ (2x)° + 54° = 180° \]
So the correct response is:
\[ (2x)° + 54° = 180° \]