2x2-ax=6a2,
x2-ax/2=3a2,
x2-ax/2+a2/16-a2/16=3a2 (completing the square: halve the x term and square the coefficient: ax/2→ax/4, coefficient (a/4)2),
(x-a/4)2=3a2+a2/16=49a2/16=(7a/4)2,
(x-a/4)2-(7a/4)2=0 (difference of two squares),
(x-a/4+7a/4)(x-a/4-7a/4)=0, so:
x-a/4+7a/4=0, and x=-3a/2 or:
x-a/4-7a/4=0, and x=2a.
Compare with factorising:
2x2-ax=6a2,
2x2-ax-6a2=(2x+3a)(x-2a), x=-3a/2 or 2a.