Question

What is the length of AC?

Answer 1

use pythagorian theorem c^2=a^2+b^2
c^2=8^2+4^2=64+16=80
\[c=\sqrt{80}\]

Answer 2

candybabe, just note that the equation is generally written \[c^2=a^2+b^2\], but in your drawing it would be \[b^2=a^2+c^2\], since sides are labeled as across from the angles.
but the numbers from eseidl are right.