I need help, what's the volume of figure 1 and 2

Which is an equation for the nth terms of the sequence 12,15,18,21

Lance lived in portugal and brazil for a total of 14 months to learn portuguese. he learned an average of 130 new words per month when he lived in portugal, and an average of 150 new words per month when he lived in brazil. in total he learned 1920 new words. write a system of equations to represent the situation. use x to represent portugal, and y to represent brazil

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