Sagot :

[tex]y^2+14y+49=0\\\\\underbrace{y^2+2\cdot y\cdot7+7^2}_{(*)}=0\\\\(y+7)^2=0\iff y+7=0\to y=-7\\\\-----------------------\\\\(*)\ \ \ (a+b)^2=a^2+2ab+b^2[/tex]