The missing statement for step 5 in the proof is:
m∠SQV + m∠VQT = m∠SQT.
This statement follows from step 4, where we applied the Angle Addition Postulate to conclude that the sum of angles \(\angle SQV\) and \(\angle VQT\) equals the straight angle \(\angle SQT\). Thus, step 5 should state that \(m\angle SQV + m\angle VQT = m\angle SQT\).