From which city did the king of Assyria send the Rabshakeh with an army to Jerusalem?

Answer: Lachish

The Rabshakeh came from Lachish to Hezekiah and stopped by the aqueduct of the upper pool, on the road to the Launderer's Field.

“And the king of Assyria sent the Rabshakeh, with a great army, from Lachish to King Hezekiah at Jerusalem. And he stopped by the aqueduct of the upper pool, on the road to the Launderer’s Field.”— Isaiah 36:2 (BSB)