If the word HEART is written as SQPMZ, then EARTH can be written as

Concept: In a letter-substitution code, once every letter of the alphabet is assigned a fixed code letter (as fixed here by how HEART becomes SQPMZ), that same individual letter-to-code pairing holds for any other word built from the same letters — a letter's position in the word does not change its code.

1. From HEART → SQPMZ, read the code position by position to fix the letter map: H → S, E → Q, A → P, R → M, T → Z.

2. EARTH is made of exactly the same five letters as HEART, only rearranged in the order E, A, R, T, H.

3. Apply the fixed map above to each letter of EARTH in that order: E → Q, A → P, R → M, T → Z, H → S.

4. Reading the resulting codes in sequence gives Q, P, M, Z, S, i.e. QPMZS.

Cross-check: Reversing the same map on Q, P, M, Z, S (Q → E, P → A, M → R, Z → T, S → H) reproduces E, A, R, T, H exactly, confirming the map was applied consistently in both directions.

So EARTH is coded as QPMZS.