a2+3a=400

Solution for a2+3a=400 equation:

a2+3a=400

We move all terms to the left:

a2+3a-(400)=0

We add all the numbers together, and all the variables

a^2+3a-400=0

a = 1; b = 3; c = -400;
Δ = b2-4ac
Δ = 32-4·1·(-400)
Δ = 1609
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(3)-\sqrt{1609}}{2*1}=\frac{-3-\sqrt{1609}}{2}
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(3)+\sqrt{1609}}{2*1}=\frac{-3+\sqrt{1609}}{2}
$