Find the Vertex f(x)=3x^2-18x+21

$f (x) = 3 x^{2} - 18 x + 21$

Step 1

Rewrite the equation in vertex form.

Complete the square for $3 x^{2} - 18 x + 21$ .

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

$a = 3$

$b = - 18$

$c = 21$

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 $d = \frac{b}{2 a}$ .

$d = \frac{- 18}{2 \cdot 3}$

Simplify the right side.

Cancel the common factor of $- 18$ and $2$ .

Factor $2$ out of $- 18$ .

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

Cancel the common factors.

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

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

Cancel the common factor.

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

Rewrite the expression.

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

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

Factor $3$ out of $- 9$ .

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

Cancel the common factors.

Factor $3$ out of $3$ .

$d = \frac{3 \cdot - 3}{3 (1)}$

Cancel the common factor.

$d = \frac{3 \cdot - 3}{3 \cdot 1}$

Rewrite the expression.

$d = \frac{- 3}{1}$

Divide $- 3$ by $1$ .

$d = - 3$

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 $e = c - \frac{b^{2}}{4 a}$ .

$e = 21 - \frac{{(- 18)}^{2}}{4 \cdot 3}$

Simplify the right side.

Simplify each term.

Raise $- 18$ to the power of $2$ .

$e = 21 - \frac{324}{4 \cdot 3}$

Multiply $4$ by $3$ .

$e = 21 - \frac{324}{12}$

Divide $324$ by $12$ .

$e = 21 - 1 \cdot 27$

Multiply $- 1$ by $27$ .

$e = 21 - 27$

Subtract $27$ from $21$ .

$e = - 6$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $3 {(x - 3)}^{2} - 6$ .

$3 {(x - 3)}^{2} - 6$

Set $y$ equal to the new right side.

$y = 3 {(x - 3)}^{2} - 6$

Step 2

Use the vertex form, $y = a {(x - h)}^{2} + k$ , to determine the values of $a$ , $h$ , and $k$ .

$a = 3$

$h = 3$

$k = - 6$

Step 3

Find the vertex $(h, k)$ .

$(3, - 6)$

Step 4

Find the Vertex f(x)=3x^2-18x+21

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