The cache is not hidden at the posted coordinates, but it is within reasonable walking distance.
First, you must complete the Adventure Lab titled Ballard's National Historic Places. As you move through the sites you'll be rewarded with the information to solve for the final location of this cache.
At each of the five locations you were given a number, A through E. Use A through E as directed to solve for x and y:
x = (A * B) + C
y = D * E
The bonus cache is hidden at:
N47 39.x
W122 22.y