Two angles are supplementary if the sum of their measures equals 180 degrees. Given \( m∠QRS = (x + 30)° \) and \( m∠SRT = (2x)° \), we can set up the equation:
\[ (x + 30) + (2x) = 180 \]
Combining like terms, we get:
\[ 3x + 30 = 180 \]
Next, we subtract 30 from both sides:
\[ 3x = 150 \]
Now, divide by 3:
\[ x = 50 \]
Now we can find the measure of \( m∠QRS \):
\[ m∠QRS = x + 30 = 50 + 30 = 80° \]
Thus, the value of \( m∠QRS \) is:
\[ \boxed{80°} \]