alrighty
we will complete the squaer for ax²+bx+c=0 regardless of the values of a,b, or c
we will factor it into a(x-h)²=r
so
first group x terms
(ax²+bx)+c=0
factor out a

take 1/2 of the linear coefinet and squaer it

square it to get

add positive and negative of that inside parntheasees

factor perfect square

expand

and that's how you complete the square, just move the constants over to the left when you're done then divide both sides by a then square root both sides, remembering to take the positive and negative roots