Let X be a r.v. has the probability function given by

If X<1  ,then F(x)=0
If 1≤X<2  ,then F(1)=P(1)=3⁄7
If 2≤X<3  ,then F(2)=P(1)+P(2)=3⁄7+2⁄7=5⁄7
If 3≤X<4  ,then F(3)=P(1)+P(2)+P(3)=6⁄7
If 4≤X  ,then F(4)=1
Thus:
F(x)={(                           0                                                        X<1                   3⁄7                                             1≤X<2     (   5)⁄7                                                    2≤X<3                  6⁄7                                                             3≤X<4 1                                                                4≤X  )