Confused as what formulas to used to solve such problem!

The trains stop 69 meters apart.

Explanation:

For each train, we're given the initial velocity, final velocity, and acceleration.  We want to find the displacement.  So we need to use an equation that's independent of time:

v² = v₀² + 2aΔx

First, convert km/h to m/s:

100 km/h = 27.8 m/s

128 km/h = 35.6 m/s

For the first train:

(0 m/s) = (27.8 m/s) + 2 (-0.9 m/s²) Δx

Δx = 429 m

For the second train:

(0 m/s) = (35.6 m/s) + 2 (-0.9 m/s²) Δx

Δx = 702 m

The first train moves 429 m before stopping, and the second train moves 702 m before stopping.  They move a total distance of 1131 m.  They were initially 1200 m apart, so they stop with 69 m to spare.

reserve is the answer