To find the equation to solve for \( x \), we start with the fact that supplementary angles sum to 180 degrees. Given that:
- \( m∠QRS = 33° \)
- \( m∠RST = (2x + 21)° \)
We can set up the equation as follows:
\[ m∠QRS + m∠RST = 180° \]
Substituting the values we have:
\[ 33° + (2x + 21)° = 180° \]
Now, combining the constants:
\[ (2x + 33 + 21)° = 180° \]
This simplifies to:
\[ (2x + 54)° = 180° \]
Therefore, the correct equation to solve for \( x \) is:
\[ (2x)° + 54° = 180° \]
So the correct response is:
(2x)° + 54° = 180°.