2. i know that but take number one for example when i get -4= +-radical(-36)/2 i don't know what to do

3. hello berserk,

i am not sure if you have learn complex number yet but,
$\sqrt {-1} = \pm i$
and
$\sqrt {-25}=
\sqrt {-5^2} = \pm 5i
$

therefore say,

$x=\sqrt{-25}$

then,

$
x=\pm 5i
$