Yoku Wakaranai keredo Isekai ni Tensei Shiteita You desu

Ren is a young girl in a primitive world of magic and brutality. Sold as a slave and about to be delivered to an unscrupulous merchant, she suddenly awakens to vivid memories of a past life: a world called Earth, where Ren was both a grown man and a research scientist. Now, she must learn to survive, combining her knowledge of science from her past life with her talent in magic from this life.

(Source: Seven Seas Entertainment)