The length of the blade of the windmill will be equal to 7 feet.
We have the equation
[tex]\rm h=7 \ Sin(\dfrac{\pi}{21t})+28[/tex]
At the time 't' = 0, the end of the blade is pointing to the right parallel to the ground meaning it is at the same height as the other end. (Ф = 0°)
So, by calculating the maximum height of this end at  Ф = 90°. we can calculate the length of the blade.
Now, we know that a general model equation of a circular simple harmonic motion is represented as
[tex]\rm y=A\ Sinwt+k[/tex]
Where A is the amplitude that is, maximum displacement from mean to maximum position.
ω is the angular frequency.
Comparing the above equations we can conclude that
A = 7
so the difference in blade's end height at Ф = 0° and Ф = 90° is 7 feet.
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