If we add 15 to both sides, we get
[tex]x^2+8x+16=15[/tex]
We can read the left hand side as
[tex]x^2 + 2\cdot 4\cdot x+4^2 = (x+4)^2[/tex]
So, the equation becomes
[tex](x+4)^2=15[/tex]
Take the square root of both sides:
[tex]x+4 = \pm\sqrt{15}[/tex]
Subtract 4 from both sides:
[tex]x=\pm\sqrt{15}-4[/tex]