This is a mystery/puzzle cache. The solution is found through completing the nearby Adventure Lab titled "Grace Murray Hopper Memorial Park."
You are solving for N38 51.ABC W077 03.DE. (In this case, E is a two-digit number.)
As you complete each step of the Adventure Lab note the partial set of coordinates that are given at the end of each answer. Once the Lab is finished you will have the full set of coordinates.
Feel free to verify your answer using the checker on this cache description page.
The cache is located on the edge of an Arlington County Park, and so the usual disclaimers apply (no night caching, etc.). You can take the walking trail from the Grace Murray Hopper Park to find it. A warning that it can be a high-muggle zone at times, but the cache is a typical hide and is right off of the sidewalk so it should be quick to sign and replace.
Enjoy!
Edit: congrats to Janomi for FTF!