Answer:
[tex]2a {}^{2} - 13a + 21 = 0 \\ {2a }^{2} - 6a - 7a + 21 = 0 \\ 2a(a - 3) - 7(a - 3) = 0 \\ (a - 3)(2a - 7) = 0 \\ a - 3 = 0 \: \: \: \: \: or \: \: \: \: \: 2a - 7 = 0 \\ \\ a = 3 \: \: \: \: or \: \: \: \: \: a = \frac{7}{2} [/tex]
