remember for ax^2+bx+c=0 first minus c from both sides ax^2+bx=-c divide both sides by a (disregard if a=1) take 1/2 of b and square it and add that to both sides factor perfect square
ok so
x^2-3x=-1 -3/2=-1.5 (-1.5)^2=2.25 add that to both sides x^2-3x+2.25=2.25-1 factor (x-1.5)^2=1.25