[tex]zad.a\\\\(-2y)\cdot (3x+y)-2x\cdot (x-y)=-6xy-2y^{2} -2x^{2} +2xy=-2x^{2} -4xy-2y^{2} \\\\-2x^{2} -4xy-2y^{2} =-2\cdot (x^{2} +2xy+y^{2} )=-2\cdot (x+y)^{2} \\\\zad.b\\\\\dfrac{3x+9}{3} -\frac{2x-9}{4} =\dfrac{3x}{3} +\dfrac{9}{3} -(\dfrac{2x}{4} -\dfrac{9}{4} )=x+3-(\dfrac{1}{2} x-2\dfrac{1}{4} )=x+3-\dfrac{1}{2} x+2\dfrac{1}{4} =\dfrac{1}{2} x+5\dfrac{1}{4}[/tex]
