$m(m - 3)(m + 3) - (m - 2)(m^2 + 2m + 4) = m(m^2 - 9) - (m^3 - 2^3) = m^3 - 9m - m^3 + 8 = -9m + 8 = -9 * -\frac{2}{3} + 8 = -3 * -2 + 8 = 6 + 8 = 14$

$(6m - n)(6m + n) - (12m - 5n)(3m + n) = (36m^2 - n^2) - (36m^2 - 15mn + 12mn - 5n^2) = 36m^2 - n^2 - 36m^2 + 15mn - 12mn + 5n^2 = 4n^2 + 3mn = n(4n + 3m) = \frac{3}{4}(4 * \frac{3}{4} + 3 * -\frac{8}{9}) = \frac{3}{4}(3 - \frac{8}{3}) = \frac{3}{4}(3 - 2\frac{2}{3}) = \frac{3}{4} * \frac{1}{3} = \frac{1}{4}$