The equation that shows the relationship between c and p is c = 3a + 2p.
If Charlie gives a books to Pete, Charlie will have c - a books left, and Pete will have p + a books.
For Charlie to have twice as many books as Pete after giving a books to Pete, the following equation must hold:
c - a = 2(p + a)
Expanding the right side of the equation, we get:
c - a = 2p + 2a
Combining like terms on the left side of the equation, we get:
c = 3a + 2p
Therefore, the equation that shows the relationship between c and p is c = 3a + 2p.