Quadratic equation is of the form
![0=ax^2+bx+c](https://img.qammunity.org/2019/formulas/mathematics/middle-school/y4kp5idv0gkiz806w92r4hd7se98iju7vf.png)
And to derive quadratic formula from quadratic equation, we need to get rid of c from right side and move it to left side .
And for that, we have to subtract c to both sides, that is
![-c = ax^2 +bx](https://img.qammunity.org/2019/formulas/mathematics/high-school/wyfezyivt9c3eivh6g0a5hodo3iexjxanf.png)
And since we do subtraction, so we have used subtraction property here.