group x terms
p(x)=(6x^2+24x)+21
factor out the quadratic coefient
p(x)=6(x^2+4x)+21
take 1/2 of linear coefient and square it
4/2=2, 2^2=4
add negative and positive inside parenthasees
p(x)=6(x^2+4x+4-4)+21
factor perfect square
p(x)=6((x+2)^2-4)+21
expand
p(x)=6(x+2)^2-24+21
p(x)=6(x+2)^2-3
