A boat travels 36 miles downstream in 4 hrs and returns to its starting point upstream in 12hrs. Sp of stream?

i just don't get what i'm doing wrong... i know the formula, but my answer is wrong..
Everyone, thank you so much!!!! This is on my final exam in like 2 days... i hate math so much, but yes... the answer was 3.

Let s = speed of stream
v = speed of boat
downstream speed = v + s
and upstream speed = v - s
4 ( v + s) = 36
12 - 4 = 8 hours to go upstream
8 ( v - s) = 36
8 ( v + s) = 72
16v = 108
v =108/16 = 27/4 = 6.75 mph
v + s = 9
s = 9 - 6.75 = 2.25 mph

Ans 2.25mph

So the boat goes 36/4 miles per hour downstream (9 mph) and
36/12 mph upstream (3 mph)

So the difference is 6 mph.

I would think that the stream would increase the boat's speed downward by 3 mph and impede it by 3 mph when it goes upstream.

So, what is the answer?

Suppose the boat can travel x miles per hour in still water. And the speed of the current is y miles per hour.

Note that when traveling downstream the speed of the boat relative to the bank is x + y. when traveling upstream the speed of the boat relative to the bank is x - y.

So we can set up a system of two equations:
4(x + y) = 36
12(x - y) = 36

simplifying them, we have
x + y = 9
x - y = 3

Adding them together, we get
2x = 12
x = 6

y = 9 - x = 9 - 6 = 3

So the speed of the current is 3 miles per hour

speed=distance/time
However you have to combine velocities of the stream and the boat when going downstream and subtract when going upstream so
36/4 = x + y where x is the boat's speed and y is the stream's speed
36/12 = x - y going downstream
Now u have a system of equations
9 = x + y and 3 = x - y use one of the many ways to solve e.g. elimination, matrix, substitution, etc.
i'll use elimination:
9+3 = x + x + y - y
12 = 2x
x=6 so the boat is moving at 6 mph now find y
9 = 6 + y so y = 3
The stream's speed is 3 mph.

let speed of boat be x
speed of stream be y
speed downstream = x+y
speed upstream = x-y

speed = dist./time

x+y =36/4 = 9 miles per hr

x-y = 36/12 = 3 miles per hr

so,
x + y =9
x - y = 3

2x = 12
x=6

x +y =9
6+y= 9
y=3 miles per hr

ans-
speed of stream = 3 miles per hr