2x方-5x+5=0
2x^2-5x=-5
(15x方-18x+9)-(-3x方+19x-31)-8x
=15x^2-18x+9+3x^2-19x+31-8x
=18x^2-45x+40
=9(2x^2-5x)+40
=9*(-5)+40
=-5