What do engineers need to ensure there is sufficient of when designing a roller coaster?

Mechanical Energy (required for each hill, loop, helix, etc.)

Remember the law of conservation of energy. As you travel on a roller coaster, your kinetic energy and gravitational potential energy changes but have the same total. Depending on the track and the friction on it, you need to make sure that the gravitational potential energy does not "overpower" its kinetic energy and that it ensures a consistent, smooth ride.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.

god i hope this !