X(x+2)=840

Solution for X(x+2)=840 equation:

X(X+2)=840

We move all terms to the left:

X(X+2)-(840)=0

We multiply parentheses

X^2+2X-840=0

a = 1; b = 2; c = -840;
Δ = b2-4ac
Δ = 22-4·1·(-840)
Δ = 3364
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{3364}=58$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(2)-58}{2*1}=\frac{-60}{2}
=-30
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(2)+58}{2*1}=\frac{56}{2}
=28
$