All edges of the following cube are 7 cm long.

Find the exact length of AG in simplest surd form.
Please directly answer the question and provide the final value, e.g., 1, 2.5, 300.
Question: All edges of the following cube are 7 cm long.

Find the exact length of AG in simplest surd form.
Please first conduct reasoning, and then answer the question and provide the final value, e.g., 1, 2.5, 300, at the end.
Question: All edges of the following cube are 7 cm long.

Find the exact length of AG in simplest surd form.