which mathematician introduced simultaneous equation?

Answer 1

Hard call. The modern notation for algebra didn't come around until the 16th century or so.

On the other hand, Diophantus looked at simultaneous equations in ancient Greece. (Diophantine Equations can be simultaneous.)

And one of the most famous theorems about simultaneous equations is the "Chinese Remainder Theorem", which if it's properly named suggests they were studied around the world.

Answer 2

Gregor Simultaneous!