7x^2+28x=-21

Solution for 7x^2+28x=-21 equation:

7x^2+28x=-21

We move all terms to the left:

7x^2+28x-(-21)=0

We add all the numbers together, and all the variables

7x^2+28x+21=0

a = 7; b = 28; c = +21;
Δ = b2-4ac
Δ = 282-4·7·21
Δ = 196
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{196}=14$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(28)-14}{2*7}=\frac{-42}{14}
=-3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(28)+14}{2*7}=\frac{-14}{14}
=-1
$