easy! that is a special triangle w/ angles 45-45-90 and sides x-x-[tex] \sqrt{x} [/tex] respectively
so, because u are given the xs of the theory i explained you above (which in this case are "8"), u only have to plug in the 8 value into the square root.
this is because, again, the diagram is a special triangle which means its sides & angles are given *u can refer to the uploaded image's first triangle
