First, square #2x^2 y^4 x# to get #4x^4 y^8 z^2#. Then, multiply it with #-2x^2 y x^3# by combining #-2# and #4# to get #-8#, #x^2# and #x^4# to get #x^6#, #y# and #y^8# to get #y^9#, and #z^3# and #z^2# to get #z^5#. Combining all of these terms gets us #-8x^6 y^9 z^5#.