Answer:
They would use terrace farming.
Step-by-step explanation:
In cases when there was not enough flat land for farming, early civilizations living in the mountain areas used the method of terrace farming.
They would cut into the slopes to make the terrace and then use the stones to support the fall. The stones also absorbed the heat of the sun, which helped the growth of the crops. Into the natural slope and the flats of it, they would plant the crops. This method was popular in Ancient China and among Incas. The terraces were around human height and can still be seen today in some places.