f(p)=(3p+7)mod26

When p= 1, substitute f(p) to get 10, that is, the letter a → j.

When p=2, substitute f(p) to get 13, that is, the letter b→ m.

When p=3, substitute f(p) to get 16, that is, the letter c → p.

When p=6, substitute f(p) to get 25, that is, the letter f → y.

When p=7, substitute f(p) to get 2, that is, the letter G → B.

When p=8, substitute f(p) to get 5, that is, the letter H → E.

When p= 15, substitute f(p) to get 0, that is, the letter o → z.

When p= 16, substitute f(p) to get 3, that is, the letter p → c.

When p= 19, substitute f(p) to get 12, that is, the letter S → L.

When p=20, substitute f(p) to get 15, that is, the letter t → o.

So stop →LOZC

Note that if the letters a, b, c, d, Z, press 0, 1, 2, 3,? Number 25

Stop →JMXA