Question

Encrypt the message need help by translating the letters into numbers, applying the encryption function f (p) = (p + 3) mod 26, and then translating the numbers back into letters.

Answer 1

This is an example of Caesar cipher!
You are learn more about that on khan academy's course on cryptography.

Here is how it works, you write a message. Then you shift every alphabet by 3 (called the key).
A = D
B = E
C = F
and so on
notice mod 26, that's for last three
X = A
Y = B
Z = C.
It works by assigning a number to every alphabet A = 1 and so on. That value is P, the argument of given function.