Solve Using the Quadratic Formula (x-2)(x+4)=7

$(x - 2) (x + 4) = 7$

Step 1

Move all terms to the left side of the equation and simplify.

Simplify the left side.

Simplify $(x - 2) (x + 4)$ .

Expand $(x - 2) (x + 4)$ using the FOIL Method.

Apply the distributive property.

$x (x + 4) - 2 (x + 4) = 7$

Apply the distributive property.

$x \cdot x + x \cdot 4 - 2 (x + 4) = 7$

Apply the distributive property.

$x \cdot x + x \cdot 4 - 2 x - 2 \cdot 4 = 7$

Simplify and combine like terms.

Simplify each term.

Multiply $x$ by $x$ .

$x^{2} + x \cdot 4 - 2 x - 2 \cdot 4 = 7$

Move $4$ to the left of $x$ .

$x^{2} + 4 \cdot x - 2 x - 2 \cdot 4 = 7$

Multiply $- 2$ by $4$ .

$x^{2} + 4 x - 2 x - 8 = 7$

Subtract $2 x$ from $4 x$ .

$x^{2} + 2 x - 8 = 7$

Subtract $7$ from both sides of the equation.

$x^{2} + 2 x - 8 - 7 = 0$

Subtract $7$ from $- 8$ .

$x^{2} + 2 x - 15 = 0$

Step 2

Use the quadratic formula to find the solutions.

$\frac{- b \pm \sqrt{b^{2} - 4 (a c)}}{2 a}$

Step 3

Substitute the values $a = 1$ , $b = 2$ , and $c = - 15$ into the quadratic formula and solve for $x$ .

$\frac{- 2 \pm \sqrt{2^{2} - 4 \cdot (1 \cdot - 15)}}{2 \cdot 1}$

Step 4

Simplify.

Simplify the numerator.

Raise $2$ to the power of $2$ .

$x = \frac{- 2 \pm \sqrt{4 - 4 \cdot 1 \cdot - 15}}{2 \cdot 1}$

Multiply $- 4 \cdot 1 \cdot - 15$ .

Multiply $- 4$ by $1$ .

$x = \frac{- 2 \pm \sqrt{4 - 4 \cdot - 15}}{2 \cdot 1}$

Multiply $- 4$ by $- 15$ .

$x = \frac{- 2 \pm \sqrt{4 + 60}}{2 \cdot 1}$

Add $4$ and $60$ .

$x = \frac{- 2 \pm \sqrt{64}}{2 \cdot 1}$

Rewrite $64$ as $8^{2}$ .

$x = \frac{- 2 \pm \sqrt{8^{2}}}{2 \cdot 1}$

Pull terms out from under the radical, assuming positive real numbers.

$x = \frac{- 2 \pm 8}{2 \cdot 1}$

Multiply $2$ by $1$ .

$x = \frac{- 2 \pm 8}{2}$

Simplify $\frac{- 2 \pm 8}{2}$ .

$x = - 1 \pm 4$

Step 5

The final answer is the combination of both solutions.

$x = 3, - 5$

Solve Using the Quadratic Formula (x-2)(x+4)=7

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