Simplifying
(5 + x)(2x + -10)

Reorder the terms:
(5 + x)(-10 + 2x)

Multiply (5 + x) * (-10 + 2x)
(5(-10 + 2x) + x(-10 + 2x))
((-10 * 5 + 2x * 5) + x(-10 + 2x))
((-50 + 10x) + x(-10 + 2x))
(-50 + 10x + (-10 * x + 2x * x))
(-50 + 10x + (-10x + 2x2))

Combine like terms: 10x + -10x = 0
(-50 + 0 + 2x2)
(-50 + 2x2)