# How do you solve the equation by graphing  3x^2 + 9x = 12?

Mar 15, 2016

You could graph the function and intersect it with a horizontal line at $y = 12$

#### Explanation:

On your GC you would use ${Y}_{1} = 3 {x}^{2} + 9 x$ and ${Y}_{2} = 12$
And then the intersect -function. Make sure you set the windows at the right values. This usually takes some trying.

Easier:
At first we can divide everything by $3$
$\to {x}^{2} + 3 x = 4$
Then subtract $4$ on both sides:
$\to {x}^{2} + 3 x - 4 = 0$
You can factor this into:
$\to \left(x - 1\right) \left(x + 4\right) = 0$
So:
$\to x - 1 = 0 \to x = 1$
$\mathmr{and} x + 4 = 0 \to x = - 4$