Hi, i'm completly at a loss as to how to approach this question and don't know how to answer it, can someone help me please.

The discrete random variable X has c.d.f. F(x) defined by

F(x) = (2+x)/7 ; x = 1, 2, 3, 4 and 5

a/ Find P(X<=3)

b/ Show that P(X=4) = 1/7

c/ Find the probability distribution for X.