21=-0.05x^2+3x+3

Solution for 21=-0.05x^2+3x+3 equation:

21=-0.05x^2+3x+3

We move all terms to the left:

21-(-0.05x^2+3x+3)=0

We get rid of parentheses

0.05x^2-3x-3+21=0

We add all the numbers together, and all the variables

0.05x^2-3x+18=0

a = 0.05; b = -3; c = +18;
Δ = b2-4ac
Δ = -32-4·0.05·18
Δ = 5.4
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{-(-3)-\sqrt{5.4}}{2*0.05}=\frac{3-\sqrt{5.4}}{0.1}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-3)+\sqrt{5.4}}{2*0.05}=\frac{3+\sqrt{5.4}}{0.1}
$