Two angles are supplementary if the sum of their measures is 180 degrees. Given:
\[ m∠QRS = (x + 40)° \] \[ m∠SRT = x° \]
We can set up the equation:
\[ m∠QRS + m∠SRT = 180° \]
Substituting the expressions for the angles:
\[ (x + 40) + x = 180 \]
Combine like terms:
\[ 2x + 40 = 180 \]
Now, subtract 40 from both sides:
\[ 2x = 180 - 40 \] \[ 2x = 140 \]
Divide both sides by 2:
\[ x = 70 \]
Now that we have the value of \(x\), we can find the measure of \(m∠QRS\):
\[ m∠QRS = x + 40 = 70 + 40 = 110° \]
Therefore, the measure of \(∠QRS\) is \( \boxed{110} \) degrees.