2b^2+6b-21=0

Solution for 2b^2+6b-21=0 equation:

2b^2+6b-21=0

a = 2; b = 6; c = -21;
Δ = b2-4ac
Δ = 62-4·2·(-21)
Δ = 204
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{204}=\sqrt{4*51}=\sqrt{4}*\sqrt{51}=2\sqrt{51}$
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(6)-2\sqrt{51}}{2*2}=\frac{-6-2\sqrt{51}}{4}
$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(6)+2\sqrt{51}}{2*2}=\frac{-6+2\sqrt{51}}{4}
$