What does x represent on a motion in physics

The variable xxx is often used to represent the horizontal position. The variable y is often used to represent the vertical The airplane passenger’s initial position is x_0=6.0\text{ m}x0=6.0 mx, start subscript, 0, end subscript, equals, 6, point, 0, space, m and his final position is x_f=2.0\text{ m}xf=2.0 mx, start subscript, f, end subscript, equals, 2, point, 0, space, m, so his displacement can be found as follows,Δx=xf−x0=2.0 m−6.0 m=−4.0 m. His displacement is negative because his motion is toward the rear of the plane, or in the negative x direction in our coordinate system.position. 

Answer sample (c); // would be best because the percentage of the energy in an incident wave that remains in a reflected wave from this material is the smallest;