Since the angles are supplementary, their measures add up to \(180^\circ\). Thus, we can set up the following equation:
\[ m∠QRS + m∠SRT = 180° \]
Substituting the given measures:
\[ (x + 40) + x = 180 \]
Combining like terms, we get:
\[ 2x + 40 = 180 \]
Next, subtract \(40\) from both sides:
\[ 2x = 180 - 40 \]
\[ 2x = 140 \]
Now, divide by \(2\):
\[ x = 70 \]
Now we can substitute \(x\) back into the expression for \(m∠QRS\):
\[ m∠QRS = x + 40 = 70 + 40 = 110° \]
Therefore, the measure of \(∠QRS\) is \(\boxed{110°}\).