Can someone explain substitution more clearly?

as in solving systems by substitution. i'm genuinely stumped, can the process be explained in a coherent and straightforward manner with examples?

If c(x) = 4x – 2 and d(x) = x2 + 5x, what is (cxd)(x)

Question: a) does the given set of points represent a linear function? how do you know? b) assume that the point (-2,4) lies on the graph of a line. what y-values could complete this set of ordered pairs that lie on the same line: (- (hint: there are several correct answ

Liz is shipping a package to her friend. the maximum weight of a medium flat-rate box is 20 pounds, including the box which weighs 6 ounces. write and solve an inequality to describe the possible weight ,w, of the contents that liz will ship to her friend if she must use a medium flat-rate box.