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Answers:</h3>

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Work Shown:
Part 1

Notice how I replaced every x with g(x) in step 2. Then I plugged in g(x) = x^2+6 and simplified.
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Part 2

In step 4, I used the rule (a+b)^2 = a^2+2ab+b^2
In this case, a = sqrt(x-1) and b = 5.
You could also use the box method as a visual way to expand out 
Answer:
(A)


(B)

(C)

(D)
The high school's enrollment in 2017 is 2500
Step-by-step explanation:
Let's assume time starts since 2009
so, In 2009 , t=0
P=900
In 2012,
t=2012-2009=3
P=1500
(A)
So, we have points as


(B)
we can use slope formula

we can plug values


we know that
slope is rate of change of P with respect to time
so, slope means increase in population is 200 per year
(C)
we can use point slope form of line





(D)
In 2017 ,
t=2017-2009=8
we can plug t=8
and we get


The high school's enrollment in 2017 is 2500
No he is wrong because 75% more time would be spent doing homework