-x- y=-4
-2x+y=-5
How do you it for solving eliminating

x = 3, y = 1

(3,1)

Step-by-step explanation:

Elimination is a method of solving systems where we add or subtract the two equations to "eliminate" one of the variables.

Take a look at our system. We have a -y and a +y. If we added those together, they would cancel!

-x- y=-4

-2x+y=-5   +

-3x + 0y = -9

It worked! No more y. Solve for x!

-3x = -9

x = 3

Plug x back in to get y.

-x - y = -4

-3 - y = -4

-y = -1

y = 1

You have to make a cos graph that starts its minimum and of -2, has an amplitude of 10, a period of 10 and  a maximum of 18.
I decided to use a cos graph since cos graphs start at their minimum or maximum unlike a sine graph that starts halfway between the minimum and maximum.  You also know the amplitude has to be 10 since 18+2=20 and 20/2=10.  We were also told that the water wheel completels a rotation every 10 minutes which means the period is 10 minutes.

lets start of with a regular cos(x) graph.  This starts on its maximum instead of minimum so we have to multiply it by -1 to get -cos(x) which does start on its minimum.

-cos(x) has an amplitude of 1 instead of 10, to fix that we multiply it by 10 to get -10cos(x) which has an amplitude of 10.
 
-10cos(x) has a period of 2π instead of 10, to fix this we multiply the x by 2π/10 to get -10cos((π/5)x) which now has a period of 10.

-10cos((π/5)x) has a minimum of -10 and maximum of 10 instead of a minimum of -2 and maximum of 18, to fix this we add 8 to -10cos((π/5)x) to get -10cos((π/5)x)+8 which does have a minimum of -2 and maximum of 18.

Therefore the answer is y=-10cos((π/5)x)+8. x being time in minutes and and y being the height in feet.  

I hope this helps.  Let me know if anything is unclear.