since the airplane is moving with constant speed then the distance travelled can be calculated by x=u*t
in km/h would be:
x=640*3.5=2.240 km
in m/s would be:
u=640 km/h= 640.000 m/h =640.000/ 3600 m/s =177.8 m/s
t= 3.5 h= 3.5 * 3600 s=12.600 s
then x=u*t= 177.8*12.600= 2.240.280 m