Help me tell what number shape to choose as an answer

In a graph with several intervals o data how does a constant interval appear? what type of scenario produces a constant interval?

Icould really use some with this question.

Frank has a devised a formula for his catering business that calculates the number of meatballs he needs to prepare. the formula is m=4a+2c, where c=number of children; m=number of meatballs; a=number of adults. how many meatballs are required for a party of 10 adults and 5 children?