With Math and Science at My Fingertips, I Can Rule Ancient Times After I Transmigrate

What should you do when you suddenly transmigrate to ancient times and can't even get enough to eat? What, the men are nearly wiped out, so the government has issued me a beautiful wife and demands we have a child next year? What, foreign tribes are invading the Central Plains again? ...No problem. Master math and science, and you can go anywhere without fear! The grassland cavalry is unstoppable? The Japanese pirates' ships are indestructible? What a joke!