100=15*x+4.9*x^2

Solution for 100=15*x+4.9*x^2 equation:

100=15x+4.9x^2

We move all terms to the left:

100-(15x+4.9x^2)=0

We get rid of parentheses

-4.9x^2-15x+100=0

a = -4.9; b = -15; c = +100;
Δ = b2-4ac
Δ = -152-4·(-4.9)·100
Δ = 2185
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{-(-15)-\sqrt{2185}}{2*-4.9}=\frac{15-\sqrt{2185}}{-9.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+\sqrt{2185}}{2*-4.9}=\frac{15+\sqrt{2185}}{-9.8}
$