The problem provides the map scale that is commonly reported as it follows: 1 cm as measured on map surface is equal to 12 Km measured on the earth's surface.
The problem provides you the distance between two points 0f 2.1 cm as measured on the surface on map and you need to determine the distance on the earth's surface such that:
distance on earth's surface = 2.1*12 = 25.2 Km
Hence, evaluating the distance on earth's surface between two points under given conditions yields 25.2 Km.
You gotta use the unitary method to do this sum.
According to the sum 1cm on the map is 12km on the ground.
The cities are 2.1 cm apart on the map.
If 1cm on the map is 12km on the ground
then 2.1cm on map would be 2.1 *12 km on the ground = 25.2km