The scale of the map is 1: 10000000 which is the distance from a to b if it is 9 cm on the map.

In order to calculate what the distance from a to b is, you need to multiply the distance on the map by the map scale. It turns out:
1) 9 cm * 10,000,000 = 90,000,000 cm.
Let’s translate the distance into kilometers.
One meter is 100 centimeters:
90,000,000 centimeters = 900,000 meters.
One kilometer is 1000 meters.
900,000 meters = 900 kilometers.
Answer: the distance from a to b is 900 kilometers.