Answer: (x-8)(x+9)
To rewrite the expression in the form (x+a)(x+b), you must factor the expression. First split the equation into two sides. Since 9 times 8 equals 72, we want to figure out how they add to equal 1 (which is the x).
If 9 times -8 equals -72, this works. -8 + 9 = 1. This translates to -8x + 9x = x. This is how we split it.
x^2 - 8x + 9x - 72
(x^2 - 8x) + (9x - 72)
Since x^2 and -8x both share x, factor out x from the parentheses. And since 9x and -72 are both divisible by 9, factor out 9.
x(x - 8) + 9(x - 8)
We can now factor out the common term, x-8. Then the x and 9 outside the parentheses will be combined.
(x-8)(x+9).
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