7x2+9x=21

Solution for 7x2+9x=21 equation:

7x^2+9x=21

We move all terms to the left:

7x^2+9x-(21)=0

a = 7; b = 9; c = -21;
Δ = b2-4ac
Δ = 92-4·7·(-21)
Δ = 669
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{-(9)-\sqrt{669}}{2*7}=\frac{-9-\sqrt{669}}{14}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(9)+\sqrt{669}}{2*7}=\frac{-9+\sqrt{669}}{14}
$