OK so we know the opposite side is 5cm and it is the hypotoneuse we want. If u refer to SOH CAH TOA the only thing that seems to satisfy that is SOH thats sin x= opposite/hypotenuse.
So sin 50 = 5cm/x
so x(sin 50)=5cm
so x=5cm/sin(50)
so x =6.53cm to 3 s.f
Hope that helped if you need any clarification just ask