7x(-3x)=-28

Solution for 7x(-3x)=-28 equation:

7x(-3x)=-28

We move all terms to the left:

7x(-3x)-(-28)=0

We add all the numbers together, and all the variables

7x(-3x)+28=0

We multiply parentheses

-21x^2+28=0

a = -21; b = 0; c = +28;
Δ = b2-4ac
Δ = 02-4·(-21)·28
Δ = 2352
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{2352}=\sqrt{784*3}=\sqrt{784}*\sqrt{3}=28\sqrt{3}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-28\sqrt{3}}{2*-21}=\frac{0-28\sqrt{3}}{-42}
=-\frac{28\sqrt{3}}{-42}
=-\frac{2\sqrt{3}}{-3}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+28\sqrt{3}}{2*-21}=\frac{0+28\sqrt{3}}{-42}
=\frac{28\sqrt{3}}{-42}
=\frac{2\sqrt{3}}{-3}
$