How do you solve #4x^2+40x+280=0# by completing the square?
You can't, unless you want to have complex numbers. If you want complex numbers, then the answer would be
You can simplify the quadratic to make completing the square easier:
Completing the square:
We cannot continue here, since there is no way for the above equation to be true.
However, if you want to have complex numbers, we can continue: