Find the Vertex Form f(x)=-3x^2-9x+26

$f (x) = - 3 x^{2} - 9 x + 26$

Step 1

Write $f (x) = - 3 x^{2} - 9 x + 26$ as an equation.

$y = - 3 x^{2} - 9 x + 26$

Step 2

Complete the square for $- 3 x^{2} - 9 x + 26$ .

Use the form $a x^{2} + b x + c$ , to find the values of $a$ , $b$ , and $c$ .

$a = - 3$

$b = - 9$

$c = 26$

Consider the vertex form of a parabola.

$a {(x + d)}^{2} + e$

Find the value of $d$ using the formula $d = \frac{b}{2 a}$ .

Substitute the values of $a$ and $b$ into the formula $d = \frac{b}{2 a}$ .

$d = \frac{- 9}{2 \cdot - 3}$

Cancel the common factor of $- 9$ and $- 3$ .

Factor $- 3$ out of $- 9$ .

$d = \frac{- 3 (3)}{2 \cdot - 3}$

Cancel the common factors.

Factor $- 3$ out of $2 \cdot - 3$ .

$d = \frac{- 3 (3)}{- 3 \cdot 2}$

Cancel the common factor.

$d = \frac{- 3 \cdot 3}{- 3 \cdot 2}$

Rewrite the expression.

$d = \frac{3}{2}$

Find the value of $e$ using the formula $e = c - \frac{b^{2}}{4 a}$ .

Substitute the values of $c$ , $b$ and $a$ into the formula $e = c - \frac{b^{2}}{4 a}$ .

$e = 26 - \frac{{(- 9)}^{2}}{4 \cdot - 3}$

Simplify the right side.

Simplify each term.

Raise $- 9$ to the power of $2$ .

$e = 26 - \frac{81}{4 \cdot - 3}$

Multiply $4$ by $- 3$ .

$e = 26 - \frac{81}{- 12}$

Cancel the common factor of $81$ and $- 12$ .

Factor $3$ out of $81$ .

$e = 26 - \frac{3 (27)}{- 12}$

Cancel the common factors.

Factor $3$ out of $- 12$ .

$e = 26 - \frac{3 \cdot 27}{3 \cdot - 4}$

Cancel the common factor.

$e = 26 - \frac{3 \cdot 27}{3 \cdot - 4}$

Rewrite the expression.

$e = 26 - \frac{27}{- 4}$

Move the negative in front of the fraction.

$e = 26 - - \frac{27}{4}$

Multiply $- - \frac{27}{4}$ .

Multiply $- 1$ by $- 1$ .

$e = 26 + 1 (\frac{27}{4})$

Multiply $\frac{27}{4}$ by $1$ .

$e = 26 + \frac{27}{4}$

To write $26$ as a fraction with a common denominator, multiply by $\frac{4}{4}$ .

$e = 26 \cdot \frac{4}{4} + \frac{27}{4}$

Combine $26$ and $\frac{4}{4}$ .

$e = \frac{26 \cdot 4}{4} + \frac{27}{4}$

Combine the numerators over the common denominator.

$e = \frac{26 \cdot 4 + 27}{4}$

Simplify the numerator.

Multiply $26$ by $4$ .

$e = \frac{104 + 27}{4}$

Add $104$ and $27$ .

$e = \frac{131}{4}$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $- 3 {(x + \frac{3}{2})}^{2} + \frac{131}{4}$ .

$- 3 {(x + \frac{3}{2})}^{2} + \frac{131}{4}$

Step 3

Set $y$ equal to the new right side.

$y = - 3 {(x + \frac{3}{2})}^{2} + \frac{131}{4}$

Find the Vertex Form f(x)=-3x^2-9x+26

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