Mario tried to solve an equation step by step: 4/3=q+1/3 step one: 4/3-1/3=q+1/3-1/3
step two: 2/3=q
find mario's mistake
A step one
b step two
c mario did not make a mistake

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

this may not be at all, but i use the same website you're using for school, and if i'm stuck doing one of these i try to look for it on the section.