[tex]a = (x - 3) {}^{2} - (x - 1)(x - 2)[/tex]
[tex]a = x {}^{2} + 3 {}^{2} - x \times 3 \times 2 - (x {}^{2} - 2x - 1x + 2)[/tex]
[tex]a = {x}^{2} + 9 - 6x - {x}^{2} + 2x + x - 2[/tex]
[tex]a = {x}^{2} - {x}^{2} - 6x + 2x + x + 9 - 2[/tex]
[tex]a = - 3x + 7[/tex]