I will share with u a simple law:
if there are m ways of doing one thing and
there are n ways of doing another thing,
in total there are:
[tex]m\times n[/tex]ways of doing the two things
Now,
3 ways of going from A to B
7 ways of going from B to C
Hence,
3 * 7 = 21 ways of going from A to C
So,
21 different paths from A to C