x^2-bx-21=(x+p)(x-7)

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Solution for x^2-bx-21=(x+p)(x-7) equation:


Simplifying
x2 + -1bx + -21 = (x + p)(x + -7)

Reorder the terms:
-21 + -1bx + x2 = (x + p)(x + -7)

Reorder the terms:
-21 + -1bx + x2 = (p + x)(x + -7)

Reorder the terms:
-21 + -1bx + x2 = (p + x)(-7 + x)

Multiply (p + x) * (-7 + x)
-21 + -1bx + x2 = (p(-7 + x) + x(-7 + x))
-21 + -1bx + x2 = ((-7 * p + x * p) + x(-7 + x))
-21 + -1bx + x2 = ((-7p + px) + x(-7 + x))
-21 + -1bx + x2 = (-7p + px + (-7 * x + x * x))
-21 + -1bx + x2 = (-7p + px + (-7x + x2))
-21 + -1bx + x2 = (-7p + px + -7x + x2)

Add '-1x2' to each side of the equation.
-21 + -1bx + x2 + -1x2 = -7p + px + -7x + x2 + -1x2

Combine like terms: x2 + -1x2 = 0
-21 + -1bx + 0 = -7p + px + -7x + x2 + -1x2
-21 + -1bx = -7p + px + -7x + x2 + -1x2

Combine like terms: x2 + -1x2 = 0
-21 + -1bx = -7p + px + -7x + 0
-21 + -1bx = -7p + px + -7x

Solving
-21 + -1bx = -7p + px + -7x

Solving for variable 'b'.

Move all terms containing b to the left, all other terms to the right.

Add '21' to each side of the equation.
-21 + 21 + -1bx = -7p + px + 21 + -7x

Combine like terms: -21 + 21 = 0
0 + -1bx = -7p + px + 21 + -7x
-1bx = -7p + px + 21 + -7x

Reorder the terms:
-1bx = 21 + -7p + px + -7x

Divide each side by '-1x'.
b = -21x-1 + 7px-1 + -1p + 7

Simplifying
b = -21x-1 + 7px-1 + -1p + 7

Reorder the terms:
b = 7 + -1p + 7px-1 + -21x-1

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