15=9.8*x^2

Solution for 15=9.8*x^2 equation:

15=9.8x^2

We move all terms to the left:

15-(9.8x^2)=0

We get rid of parentheses

-9.8x^2+15=0

a = -9.8; b = 0; c = +15;
Δ = b2-4ac
Δ = 02-4·(-9.8)·15
Δ = 588
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:

$\sqrt{\Delta}=\sqrt{588}=\sqrt{196*3}=\sqrt{196}*\sqrt{3}=14\sqrt{3}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-14\sqrt{3}}{2*-9.8}=\frac{0-14\sqrt{3}}{-19.6}
=-\frac{14\sqrt{3}}{-19.6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+14\sqrt{3}}{2*-9.8}=\frac{0+14\sqrt{3}}{-19.6}
=\frac{14\sqrt{3}}{-19.6}
$