12=18.4x+4.9x^2

Solution for 12=18.4x+4.9x^2 equation:

12=18.4x+4.9x^2

We move all terms to the left:

12-(18.4x+4.9x^2)=0

We get rid of parentheses

-4.9x^2-18.4x+12=0

a = -4.9; b = -18.4; c = +12;
Δ = b2-4ac
Δ = -18.42-4·(-4.9)·12
Δ = 573.76
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-18.4)-\sqrt{573.76}}{2*-4.9}=\frac{18.4-\sqrt{573.76}}{-9.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-18.4)+\sqrt{573.76}}{2*-4.9}=\frac{18.4+\sqrt{573.76}}{-9.8}
$