Deduce which number fits the position of [[$ x $]].

a) [[$ \sqrt {12 + x} = 4 $]]

[[$ x = $]]

b) [[$ \sqrt {x – 5} = 5 $]]

[[$ x = $]]

c) [[$ \sqrt {2x} = 4 $]]

[[$ x = $]]

d) [[$ 2\sqrt {x + 3} = –6 $]]

[[$ x = $]]