x2+15x+21=0

Solution for x2+15x+21=0 equation:

x2+15x+21=0

We add all the numbers together, and all the variables

x^2+15x+21=0

a = 1; b = 15; c = +21;
Δ = b2-4ac
Δ = 152-4·1·21
Δ = 141
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{-(15)-\sqrt{141}}{2*1}=\frac{-15-\sqrt{141}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+\sqrt{141}}{2*1}=\frac{-15+\sqrt{141}}{2}
$