Worth 19
a wooden water wheel is next to an old stone mill. the water wheel makes ten revolutions
every minute, dips down two feet below the surface of the water, and at its highest point is
18 feet above the water. a snail attaches to the edge of the wheel when the wheel is at its
lowest point and rides the wheel as it goes round and around. as time passes, the snail
rises up and down, and in fact, the height of the snail above the surface of the water varies
sinusoidally with time. use this information to write the particular equation that gives the
height of the snail over time.

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.

What do you mean by that?