[tex] \frac{3x + 1}{x + 3} = \frac{3x + 1}{3 + x} = \frac{(3x + 1)(3 - x)}{(3 + x)(3 - x)} = \frac{9x + 3 - 3 {x}^{2} - x}{ {3}^{2} - {x}^{2} } = \\ \frac{8x - 3 {x}^{2} + 3 }{9 - {x}^{2} } [/tex]
