Question 82350
move constant terms to right side ... {{{x^2+6x=40}}}

divide the coefficient of the x term by 2, square it, and add to both sides ... {{{x^2+6x+3^2=40+3^2}}}


factor the quadratic ... {{{(x+3)^2=49}}}


take the square root of both sides ... x+3=7 and x+3=-7 ... so x=4 and x=-10