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 = 20x
(50 + 20x + 2x2)