Question 629609
{{{ x^2 + 120x + 51 = 0 }}}
{{{ x^2 + 120x = -51 }}}
{{{ x^2 + 120x + (120/2)^2 = -51 + (120/2)^2 }}}
{{{ x^2 + 120x + 3600 = -51 + 3600 }}}
3600 completes the square