\[y = \frac{x^{2} + 2x - 3}{x + 3} = \frac{(x + 3)(x - 1)}{x + 3} = x - 1\]
\[x^{2} + 2x - 3 = (x + 3)(x - 1).\]
\[x_{1} + x_{2} = - 2;\ \ \ x_{1} \cdot x_{2} = - 3\]
\[x_{1} = - 3;\ \ x_{2} = 1.\]
\[y = x - 1;\ \ где\ x \neq - 3.\]