Simplifying
2(a2 + a)(3a2 + 6a)

Reorder the terms:
2(a + a2)(3a2 + 6a)

Reorder the terms:
2(a + a2)(6a + 3a2)

Multiply (a + a2) * (6a + 3a2)
2(a(6a + 3a2) + a2(6a + 3a2))
2((6a * a + 3a2 * a) + a2(6a + 3a2))
2((6a2 + 3a3) + a2(6a + 3a2))
2(6a2 + 3a3 + (6a * a2 + 3a2 * a2))
2(6a2 + 3a3 + (6a3 + 3a4))

Combine like terms: 3a3 + 6a3 = 9a3
2(6a2 + 9a3 + 3a4)
(6a2 * 2 + 9a3 * 2 + 3a4 * 2)
(12a2 + 18a3 + 6a4)