What is the stream gradient of GW Creek measured from the point at which it crosses the 20 ft. contour to the point at which the stream is 10 ft above sea level

The stream gradient = 5 feet/miles

Explanation:

Steam gradient formula is =  

the difference in elevation between two points on the stream over the  

distance along the stream which is measured in feet/mile, inch / km etc.

The change in stream elevation- 10 feet.  

The stream distance (shown in the map)- 2 miles  

Therefore, stream gradient- Change in stream elevation/stream distance  

The Stream gradient= 10 feet/ 2miles= 5 feet/miles.

reflecting point a across the origin negates both of its coordinates.

b = (5, -4)