[The Summoner of Leviathan]

sqrt(x) = sqrt(x-8)+2

Square both sides.

x= x-8+4sqrt(x-8)+4

Simplify

x= x-4+4sqrt(x-8)

Simplify more

4=4sqrt(x-8)

Divide everything by 4.

1=sqrt(x-8)

Square it again.

1=x-8

Add 8 to both sides.

9=x