5(-x2+3y2)-1/2[5xy+3(y2-yx+2/3x2)]
=-5x2+15y2-5/2xy-3/2(y2-yx+2/3x2)
=-5x2+15y2-5/2xy-3/2y2+3/2xy-x2
=-6x2+27/2y2-xy
=-24+27/2-2
=-25/2