Question 437260
Add +1 on both sides of equation.

{{{x^2+2x+1=15+1}}}, complete the square on the left side.

{{{(x+1)^2=16}}}, by the square root definition we write: x+1=+/- sqrt(16)

x+1=4 => x=3 and x+1=-4 => x=-5.

Done.