Simplifying
(x + -3)(10x + -20)

Reorder the terms:
(-3 + x)(10x + -20)

Reorder the terms:
(-3 + x)(-20 + 10x)

Multiply (-3 + x) * (-20 + 10x)
(-3(-20 + 10x) + x(-20 + 10x))
((-20 * -3 + 10x * -3) + x(-20 + 10x))
((60 + -30x) + x(-20 + 10x))
(60 + -30x + (-20 * x + 10x * x))
(60 + -30x + (-20x + 10x2))

Combine like terms: -30x + -20x = -50x
(60 + -50x + 10x2)