Simplifying
12a(a + b)(a + -1b)

Multiply (a + b) * (a + -1b)
12a(a(a + -1b) + b(a + -1b))
12a((a * a + -1b * a) + b(a + -1b))

Reorder the terms:
12a((-1ab + a2) + b(a + -1b))
12a((-1ab + a2) + b(a + -1b))
12a(-1ab + a2 + (a * b + -1b * b))
12a(-1ab + a2 + (ab + -1b2))

Reorder the terms:
12a(-1ab + ab + a2 + -1b2)

Combine like terms: -1ab + ab = 0
12a(0 + a2 + -1b2)
12a(a2 + -1b2)
(a2 * 12a + -1b2 * 12a)

Reorder the terms:
(-12ab2 + 12a3)
(-12ab2 + 12a3)