Can someone explain how to find the area below a curve?

:i need some real : a store sells shirts to the public at one pricing scale and wholesale at another pricing scale. the tables below describe the cost, y, of x shirts. (after tables) how do the slopes of the lines created by each table compare? the slope of the public table is 3/4 of the slope of the wholesale table. the slope of the wholesale table is 3/4 of the slope of the public table. the slope of the public table is 12 times greater than the slope of the wholesale table. the slope of the wholesale table is 12 times greater than the slope of the public table.