Алгебра: А) 9a²b (7a²-5ab-4b²) б) (5a-2b)*(3a+4b) в) (4a³b²-12a²b³):(2ab) ПОМОГИТЕ

Решите задачу:

$a)9 {a}^{2} b(7a {}^{2} - 5ab - 4b {}^{2} ) = 63 {a}^{4} b - 45 {a}^{3} b {}^{2} - 36 {a}^{2} {b}^{3 } \\ b)(5a - 2b)(3a + 4b) = 15 {a }^{2} + 20ab - 6ab - 8 {b}^{2} = 15 {a }^{2} + 14ab - 8 {b}^{2} \\$
$v)(4 {a}^{3} b {}^{3} - 12 {a}^{2} b {}^{3} ) \div (2ab) = \frac{4 {a}^{3} b {}^{3} - 12 {a}^{2} b {}^{3} }{2ab} = \frac{4 {a}^{2} {b}^{3}(a - 3b )}{2ab} = \frac{2ab {}^{2} (a - 3b)}{1} = 2ab {}^{2} (a - 3b)$