x^2+7=0
this can be written as:
x^2+0x+7=0
To solve using completer square we proceed as follows:
x^2+0x=-7
but
c=(b/2a)^2
c=(0/2*1)^2=0
hence
x^2+0x+0=-7+0
this will be simplified as:
x^2=-7
getting the square root of both sides gives us:
sqrt(x^2)=sqrt(-7)
x=+/-√-7
but √-1=i
hence our answer will be:
x=+/-7i