To solve for \( x \), we need to recognize that supplementary angles add up to \( 180^\circ \). Given that \( m∠QRS = 33^\circ \) and \( m∠RST = (2x + 21)^\circ \), we can set up the equation:
\[ m∠QRS + m∠RST = 180^\circ \]
Substituting the known values:
\[ 33^\circ + (2x + 21)^\circ = 180^\circ \]
Now, simplify this:
\[ 33 + 2x + 21 = 180 \]
Combining like terms:
\[ 2x + 54 = 180 \]
Now, isolate \( 2x \):
\[ 2x = 180 - 54 \] \[ 2x = 126 \]
Thus, the correct equation to set up from the problem statement for solving \( x \) is:
\[ (2x)^\circ + 54^\circ = 180^\circ \]
So the correct response is:
\[ (2x)° + 54° = 180° \]