P(x)=3x^2+5x-28

Solution for P(x)=3x^2+5x-28 equation:

(P)=3P^2+5P-28

We move all terms to the left:

(P)-(3P^2+5P-28)=0

We get rid of parentheses

-3P^2+P-5P+28=0

We add all the numbers together, and all the variables

-3P^2-4P+28=0

a = -3; b = -4; c = +28;
Δ = b2-4ac
Δ = -42-4·(-3)·28
Δ = 352
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$P_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$P_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{352}=\sqrt{16*22}=\sqrt{16}*\sqrt{22}=4\sqrt{22}$
$P_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-4)-4\sqrt{22}}{2*-3}=\frac{4-4\sqrt{22}}{-6}
$
$P_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-4)+4\sqrt{22}}{2*-3}=\frac{4+4\sqrt{22}}{-6}
$