Any easy ways to remember how to find the equilibrium force?

Yes.  'Equilibrium' means that the SUM of adding up all the forces acting on the object is ZERO.  

If you add up all the forces acting on the object and the sum is NOT zero, then the object is NOT in equilibrium.  The force you have to add to it is whatever it takes to cancel out (balance) all the forces that are there now.

Take the SUM you got when you added up all the existing forces.  To get equilibrium, ADD ON one new force with the SAME magnitude, in exactly the OPPOSITE direction.

answer: i'm not sure which is which for i get confused on it all the time but i can tell you that the answer lies with the convex and concave devices. i hope this you. have a good day and best of luck to you!

explanation:

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.