Question

insert one comma to separate the coordinate adjectives.
when the warped unstable shelves in the pantry began to bow in the middle, ernesto bought some new plywood to rebuild them.

Explanation:

To separate the coordinate adjectives "warped" and "unstable" (which both describe "shelves" and can be reordered or have "and" between them), we insert a comma between them.

Step1: Identify coordinate adjectives

The adjectives "warped" and "unstable" are coordinate (they modify the same noun "shelves" and are of equal importance).

Step2: Insert comma

Place a comma between "warped" and "unstable".

Answer:

When the warped, unstable shelves in the pantry began to bow in the middle, Ernesto bought some new plywood to rebuild them.