Explanation

Whenever you see exponents, think common quadratics.

If you factor the 6 out of the numerator in Quantity A, you get \(6(a^2 + 2ab + b^2)\), which includes a common quadratic \((a +b)^2\).

Then you can cancel (a + b) from both the numerator and the denominator; Quantity A is really just 6(a + b). The quantities are equal.
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