15=4.9x^2-7.5x

Solution for 15=4.9x^2-7.5x equation:

15=4.9x^2-7.5x

We move all terms to the left:

15-(4.9x^2-7.5x)=0

We get rid of parentheses

-4.9x^2+7.5x+15=0

a = -4.9; b = 7.5; c = +15;
Δ = b2-4ac
Δ = 7.52-4·(-4.9)·15
Δ = 350.25
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{-(7.5)-\sqrt{350.25}}{2*-4.9}=\frac{-7.5-\sqrt{350.25}}{-9.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(7.5)+\sqrt{350.25}}{2*-4.9}=\frac{-7.5+\sqrt{350.25}}{-9.8}
$