4=70-0.1x-0.001x^2

Solution for 4=70-0.1x-0.001x^2 equation:

4=70-0.1x-0.001x^2

We move all terms to the left:

4-(70-0.1x-0.001x^2)=0

We get rid of parentheses

0.001x^2+0.1x-70+4=0

We add all the numbers together, and all the variables

0.001x^2+0.1x-66=0

a = 0.001; b = 0.1; c = -66;
Δ = b2-4ac
Δ = 0.12-4·0.001·(-66)
Δ = 0.274
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{-(0.1)-\sqrt{0.274}}{2*0.001}=\frac{-0.1-\sqrt{0.274}}{0.002}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0.1)+\sqrt{0.274}}{2*0.001}=\frac{-0.1+\sqrt{0.274}}{0.002}
$