A car travels a distance of 840 miles at a uniform speed.

First equation: Speed = 840/Time

If the speed of the car is increased 10 m/h more, it takes two hours less to cover the same distance,

Second equation: (Speed + 10) = 840/(Time - 2)

the original speed of the car was?

So now you have two equations and two unknowns, and you have probably learned how to solve this system.

options:75 m/h,65 m/h,60 m/h,55 m/h,45 m/h.

This gives you a second way to solve it: Put each Speed option in the first equation, which gives you a corresponding Time option. Put this Speed in Time in the second equation, to see if it balances.
If not, it is the wrong Speed.

Claude