#### Answer

$x=2$

#### Work Step by Step

Square both sides and put all terms on one side to create a quadratic equation. Then use trial and error to factor it, and set the values found equal to zero. One of the solutions does not work.
$\sqrt {20-8x}=x$
$(\sqrt {20-8x})^2=x^2$
$20-8x=x^2$
$x^2+8x-20=0$
$(x+10)(x-2)=0$
$x=-10,2$
$\sqrt {20-8\times(-10)}\ne-10$
$x=2$