The length of the segment on the terrain is 4.5 km. What is the length of this segment on the map made at a scale of 1: 1,000,000.

In this case, we will carry out the reverse operations.
Let’s translate the 4.5 km segment on the ground, first in meters = 4500 m, and then in centimeters = 450,000 cm.
Because the map is made at a scale of 1: 1,000,000, then we divide the result (450,000 cm) by 1,000,000, and we get the length of the segment on the map at a scale of 1: 1,000,000 = 0.45 cm. (or 45 mm)