Answer
$4-3xy^{2}$
Work Step by Step
$5x^{2}y+6xy^{2}-5yx^{2}+4-9y^{2}x$
Rewrite the expression putting all the $x$'s first in every term:
$5x^{2}y+6xy^{2}-5x^{2}y+4-9xy^{2}=...$
Simplify by combining like terms:
$...=(5-5)x^{2}y+(6-9)xy^{2}+4=...$
$...=(0)x^{2}y-3xy^{2}+4=...$
$...=4-3xy^{2}$