In a certain code language, "FIRMLY" is written as "TKHWJK". How is "PUBLIC" written in that code language?

Let's decipher the relation between "FIRMLY" and "TKHWJK" by looking at shifts from the end or beginning:
Let's reverse the code "TKHWJK" → "KJWHKT" and match with "FIRMLY":

F
+2

​
H

I
+2

​
K

R
+2

​
T

M
−2

​
K

L
−2

​
J

Y
−2

​
W

Let's look at a simpler direct mapping:

F(6)
+14

​
T(20)

I(9)
+2

​
K(11)

R(18)
−10

​
H(8)
Let's look at alternative mappings such as cross coding:
Divide FIRMLY into two halves: FIR and MLY.
Reverse FIR → RIF. Add +2: R+2=T, I+2=K, F+2=H. This gives TKH.
Reverse MLY → YLM. Subtract −2: Y−2=W, L−2=J, M−2=K. This gives WJK.
Combined: TKHWJK.

Let's apply this exact scheme to PUBLIC:

Divide into two halves: PUB and LIC.

First half reversed: PUB→BUP. Add +2 to each letter:

B+2=D

U+2=W

P+2=R
This gives DWR.

Second half reversed: LIC→CIL. Subtract −2 from each letter:

C−2=A

I−2=G

L−2=J
This gives AGJ.

Combining both parts gives DWRAGJ.