Hint:
$$8-2\sqrt{15}=\cdots=(\sqrt5-\sqrt3)^2$$
We know for real $a,$ $$\sqrt{a^2}=|a|$$
which $=a,$ if $a\ge0$
else $=-a$
Now just multiply out
Hint:
$$8-2\sqrt{15}=\cdots=(\sqrt5-\sqrt3)^2$$
We know for real $a,$ $$\sqrt{a^2}=|a|$$
which $=a,$ if $a\ge0$
else $=-a$
Now just multiply out