\(2,\\ a,x\left(x-y\right)+y\left(x+y\right)=x^2-xy+xy+y^2=x^2+y^2=\left(-6\right)^2+8^2=100\\ b,x\left(x^2-y\right)-x^2\left(x+y\right)+y\left(x^2-x\right)\\ =x^3-xy-x^3-x^2y+xy\left(x-1\right)\\ =-xy\left(x+y\right)+xy\left(x-1\right)\\ =xy\left(x-1-x-y\right)\\ =-xy\left(1+y\right)\\ =-\dfrac{1}{2}\cdot\left(-100\right)\left(1-100\right)\\ =50\cdot\left(-99\right)=-4950\)
\(3,\\ a,3x\left(12x-4\right)-9x\left(4x-3\right)=30\\ \Leftrightarrow36x^2-12x-36x^2+27x=30\\ \Leftrightarrow15x=30\\ \Leftrightarrow x=2\\ b,x\left(5-2x\right)+2x\left(x-1\right)=15\\ \Leftrightarrow6x-2x^2+2x^2-2x=15\\ \Leftrightarrow4x=15\\ \Leftrightarrow x=\dfrac{15}{4}\)
