Let X be the variable used to represent the expression of the total number of books that Mario has read. We used 4X to represent "four times the number of books". Then we add -2 to interpret the words "for two less than". Therefore, we have an expression of 4X-2 for the total number of books that Mario has read.
(a) = 400 (b) (c) Only the positive solution makes sense because you cannot have a negative side length. (d) Since one side of the painting is 20 inches, all four sides together will be 20*4 = 80 inches