2x^2*x^3+x*(-3x^4)

=2x^(2+3)-3x^(1+4)

=2x^5-3x^5

=-x^5

(2a^2b)^2*3b-4a^4b^3

=4a^4b^2*3b-4a^4b^3

=4a^4b^3-4a^4b^3

=0