15=20-4.9x^2

Solution for 15=20-4.9x^2 equation:

15=20-4.9x^2

We move all terms to the left:

15-(20-4.9x^2)=0

We get rid of parentheses

4.9x^2-20+15=0

We add all the numbers together, and all the variables

4.9x^2-5=0

a = 4.9; b = 0; c = -5;
Δ = b2-4ac
Δ = 02-4·4.9·(-5)
Δ = 98
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{98}=\sqrt{49*2}=\sqrt{49}*\sqrt{2}=7\sqrt{2}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-7\sqrt{2}}{2*4.9}=\frac{0-7\sqrt{2}}{9.8}
=-\frac{7\sqrt{2}}{9.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+7\sqrt{2}}{2*4.9}=\frac{0+7\sqrt{2}}{9.8}
=\frac{7\sqrt{2}}{9.8}
$