Question 1138789
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point  (13, 18.8)

slope  -0.4


    y - 18.8 = -0.4*(x-13)

    y = -0.4*(x-13) + 18.8


After 21 hours the height will be

    y = -0.4*(21 - 13) + 18.8 = 15.6 centimeters.    <U>ANSWER</U>
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Notice that if you evaluate the height using formula from the @josgarithmetic post


    y = -4*(x-13) + 18.8 


you will get a negative height (!)


It is why I placed my solution here.
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