The equation for this sequence is:

a_n = a_n-1 + 2(n - 1)

where n is the term's order in the sequence, a_n is the value of the term, and a_n-1 is the value of the term before a_n. This makes the next number:

a₆ = a₅ + 2(6 - 1)
a₆ = 21 + 2(5)
a₆ = 31

First, solve for \( \left|a - 9\right| \):

\( \left|a - 9\right| \) + 2 = 1
\( \left|a - 9\right| \) = 1 - 2
\( \left|a - 9\right| \) = -1

The value inside the absolute value brackets can be either positive or negative so (a - 9) must equal - 1 or --1 for \( \left|a - 9\right| \) to equal -1:

So, a = 10 or a = 8.

The factors of a number are all positive integers that divide evenly into the number. The factors of 28 are 1, 2, 4, 7, 14, 28.

To add or subtract terms with exponents, both the base and the exponent must be the same. In this case they are so subtract the coefficients and retain the base and exponent:

6a³ - 3a³
(6 - 3)a³
3a³

To multiply terms with exponents, the base of both exponents must be the same. In this case they are so multiply the coefficients and add the exponents:

3x³ x 4x⁶
(3 x 4)x^{(3 + 6)}
12x⁹