a) \(\frac{x\left(y-x\right)+y\left(x-y\right)}{3y^2-3x^2}=\frac{x\left(y-x\right)-y\left(y-x\right)}{3\left(y^2-x^2\right)}\)
\(=\frac{\left(y-x\right)\left(x-y\right)}{3\left(y-x\right)\left(y+x\right)}\)
\(=\frac{x-y}{3\left(x+y\right)}\)
b) \(\frac{2x^2-xy-3y^2}{2x^2-5xy+3y^2}=\frac{2x^2+2xy-3xy-3y^2}{2x^2-2xy-3xy+3y^2}\)
\(=\frac{\left(2x^2+2xy\right)-\left(3xy+3y^2\right)}{\left(2x^2-2xy\right)-\left(3xy-3y^2\right)}\)
\(=\frac{2x\left(x+y\right)-3y\left(x+y\right)}{2x\left(x-y\right)-3y\left(x-y\right)}\)
\(=\frac{\left(x+y\right)\left(2x-3y\right)}{\left(x-y\right)\left(2x-3y\right)}\)
\(=\frac{x+y}{x-y}\)
