Question 791353
You don't add -1 to 15.  You add 1 to both sides.  The goal is to isolate x, while maintaining balance.  An equation is like a scale, or balance, with both sides the same.  If you want to remove -1 from the left side, you have to add 1.  -1 + 1 = 0.  If you add 1 to the left, you have to add 1 to the right to maintain the balance.<P>
{{{x^2 - 1 = 15}}}<P>
Add one to both sides.
{{{x^2 - 1 + 1 = 15 +1}}}<P>
{{{x^2 = 16}}}  Square root both sides.<P>
{{{sqrt(x^2)}}} = + or - {{{sqrt(16)}}}<P>
x = + or - 4.<P>
-4 * -4 = 16 and 4 * 4 = 16.  When you square root both sides, you have to remember it's + or - square root.