Answer:
Speed of river's current = 5 mph
Step-by-step explanation:
Let 'd' be the distance covered
Let x be the speed of the rivers current
Speed upstream = 15-x
speed downstream = 15+x
Time = distance / speed
time upstream = 
time downstream = 
the ferry trip upstream takes twice as long as its return trip downstream
= 
divide both sides by d
= 
cross multiply


Add 2x on both sideds

subtract 15 from both sides and divide by 3

x=5
Speed of river's current = 5 mph
Has to be greater than 90 degrees.
Answer:
I believe your answer is -7/54
2x + y = x - y - 3 = 6
Add 3 to everything
2x + y + 3 = x - y - 3 + 3 = 6 + 3
2x + y + 3 = x - y = 9
2x + y + 3 = 9
2x + y = 6 = x - y
Add y to everything
2x + 2y = 6 + y = x
Since x = 6 + y substitute every x with 6 + y
2(6 + y) + 2y = x
12 + 4y = x
This is as simplified as I can get and then just place 12 + 4y into every x in the original problem then solve