To answer this we apply the Pythagorean Theorem, this states that given a right triangle with a hypotenuse -longest side- (
![a](https://img.qammunity.org/2019/formulas/mathematics/middle-school/29wl7w45fjbhkymu53hsles9qvadja8y1s.png)
) and two sides (
![b](https://img.qammunity.org/2019/formulas/mathematics/college/myv2xpbx0xoe5ipweiedhwixh3h7ydfst0.png)
and
![c](https://img.qammunity.org/2019/formulas/mathematics/middle-school/9noujvlad80s1vjnwm1kln023au43wyqwx.png)
), then:
![a^2=b^2+c^2](https://img.qammunity.org/2019/formulas/mathematics/college/wrcl2bvfftp3jlm2d1gzc8ub6nz53042tl.png)
So, for your problem, we can see right triangle with a hypotenuse of 8 (
![a=8](https://img.qammunity.org/2019/formulas/mathematics/college/940afkomz18fvid2mficef184bol1fvdiz.png)
), one side equal to 4 (
![b=4](https://img.qammunity.org/2019/formulas/mathematics/college/6tw7zrs7l3dopfz9axqapv7ftynh41vzv8.png)
) and other side unknown; this means we have the following:
![8^2=4^2+x^2](https://img.qammunity.org/2019/formulas/mathematics/college/8n3uqxuorapzmoeouedr1k49kbucxcr420.png)
We can easily solve for
![x](https://img.qammunity.org/2019/formulas/mathematics/college/lhtxftojjkzsmo3o2h4ilq8naohracejui.png)
(altitude of the equilateral triangle):
Hence, the answer is E.