21=x+4.905(x^2)

Solution for 21=x+4.905(x^2) equation:

21=x+4.905(x^2)

We move all terms to the left:

21-(x+4.905(x^2))=0

We get rid of parentheses

-4.905x^2-x+21=0

We add all the numbers together, and all the variables

-4.905x^2-1x+21=0

a = -4.905; b = -1; c = +21;
Δ = b2-4ac
Δ = -12-4·(-4.905)·21
Δ = 413.02
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{-(-1)-\sqrt{413.02}}{2*-4.905}=\frac{1-\sqrt{413.02}}{-9.81}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+\sqrt{413.02}}{2*-4.905}=\frac{1+\sqrt{413.02}}{-9.81}
$