7=-0.004x^2+1.25x+15

Solution for 7=-0.004x^2+1.25x+15 equation:

7=-0.004x^2+1.25x+15

We move all terms to the left:

7-(-0.004x^2+1.25x+15)=0

We get rid of parentheses

0.004x^2-1.25x-15+7=0

We add all the numbers together, and all the variables

0.004x^2-1.25x-8=0

a = 0.004; b = -1.25; c = -8;
Δ = b2-4ac
Δ = -1.252-4·0.004·(-8)
Δ = 1.6905
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{-(-1.25)-\sqrt{1.6905}}{2*0.004}=\frac{1.25-\sqrt{1.6905}}{0.008}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1.25)+\sqrt{1.6905}}{2*0.004}=\frac{1.25+\sqrt{1.6905}}{0.008}
$