5.3
Step-by-step explanation:
a² + b² =² c²        the Pythagorean theorem
RQ² + RS² = QS²       Solve RS
6² + RS² = 8²
RS² = 64 -36
RS² = 28
RS = √28          28 is slightly > 25  and since  √25 =5
RS = 5.3 Â Â Â Â Â Â Â Â Â Â actually 5.2915 Â to a few more significant digits