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Answer: 
Step-by-step explanation:
We have the following function:

And we have to evaluate it when
, this means we have to substitute
by
:

Solving:

Finally:

Judging by the question you have provided I came to the conclusion that you have already solved your own problem!
If the goal is to find X when X=-15 then your answer for X should be -15!
If this is not the entire equation please post the entire one!
Hope this helped!
-Blake
Answer:
3
Step-by-step explanation:
Period of a function is the period after which the function attains the same value
in the graph attached with this problem we can see that
f(0)=1
the value of x for which function f(x) attains the value 1 again is at
x=3
f(3)=1
similarly , we see
f(6)=1 , f(9)=1
Hence we see that after every increased value of x by 3 units , we attain the same value of function . hence the period of the function is 3
1 foot = 12 inches, so now you add 3 inches until you get 12. you add 3, 4 times toget 12. 3+3=6+3=9+3=12 and do that again for the 2nd foot 3+3=6+3=9+3=12 you added 3, 4 times to get 12 twice so now add 4+4=8
YOUR ANSWER IS 8