finitely generated algebra, which is also a field. a well-known result in commutative algebra says that a finitely generated domain over a field F is a field iff it's algebraic over F**. so
must be an algebraic extension of which is possible only if because is algebraically closed. now let be the image of under the natural projection
then clearly and thus but we already proved that is a maximal ideal of therefore
* if is any algebraically closed field, then maximal ideals of polynomial ring are exactly the ideals
** see for example page 162 of the book "Graduate Algebra: Commutative View". the author is Louis Halle Rowen.