7m(m-3)=28

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

7m(m-3)=28

We move all terms to the left:

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

We multiply parentheses

7m^2-21m-28=0

a = 7; b = -21; c = -28;
Δ = b2-4ac
Δ = -212-4·7·(-28)
Δ = 1225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$m_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$m_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1225}=35$
$m_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-35}{2*7}=\frac{-14}{14}
=-1
$
$m_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+35}{2*7}=\frac{56}{14}
=4
$