I Am in the Tang Dynasty, and I Have Already Been Expelled from School

Li Zheng is kicked out of a study hall in Chang’an for being too stupid. Many people think there’s something wrong with him.

But when he solves the Nine Chapters of Mathematical Arts and cracks the crisis at Yinshan, everyone in Chang’an starts looking for him…

Poetry, mathematics, medicine: Li Zheng makes a name for himself in Chang’an.

The Five Great Clans try to recruit him, the emperor invites him to court, and the princes vie for his support.

With fame and fortune at his feet, Li Zheng instead withdraws from the world in full view of everyone.

…

Fang Xuanling: “Li Zheng, the situation at court is dire. Please come out of retirement.”

Li Chunfeng: “Li Zheng, I’ve given you the Tui Bei Tu. Please come out of retirement.”

Cheng Yaojin: “Li Zheng, the Turks and Tubo are invading again. Hurry up and come out of retirement.”

Li Shimin: “Li Zheng, my Great Tang seems to be on the verge of collapse. Please come out of retirement at once.”

…

Li Zheng says calmly, “Stop making such a racket! My railway is almost finished.”

This book is also known as "I Really Don’t Have the Money to Protect the Great Tang Anymore".