## anonymous 5 years ago $x=\sqrt{42-x}$

Square both sides we have $$x^2=42-x$$ $$x^2+x-42=0$$ $$(x+7)(x-6)=0$$ $$x=-7$$ or $$x=6$$ But if we plu gin $$x=-7$$ to the original equation it doesn't work! So $$x=6$$