When solving a quadratic equation, such as this, we need to factorize first:
x^2+6x+8 = 0
=> (x+4)(x+2) = 0
So x = -4 or x = -2
Why can't we do x(x+6) = -8?
And so x = -8 or x = -6 (obviously wrong)
Shucks, learnt this in sec sch but cant remeber the reason... only know it's illegal unless the RHS is a zero to begin with...