complete theh square
isolate x terms
factor out coefient of x^2 term
take 1/2 of 1st degree coefient and square it, add positive and negative inside parnthasees, expand
f(x)=(2x^2+12x)+7
f(x)=2(x^2+6x)+7
6/2=3, 3^2=9
f(x)=2(x^2+6x+9-9)+7
f(x)=2((x+3)^2-9)+7
f(x)=2(x+3)^2-19+7
f(x)=2(x+3)^2-12