Answer:
Third option. 
Step-by-step explanation:
For this exercise you need to remember one of the properties for exponents.
There is a property called the "Negative property of exponents" which states the following:
Where 
As you can observe,
is the reciprocal of 
In this case you have the following expression given in the exercise:

Observe the expression. As you can notice, the base "n" has a negative exponent, which is -6.
Therefore, applying the Negative property of exponents explained at the beginning of this explanation, you can simplify the expression.
Then, the simplified form of
is the one shown below:

3/4 - 5/9 .....common denominator is 36
27/36 - 20/36 =
7/36 <==
Answer:
D 36
Step-by-step explanation:
<em>Write the information algebraically</em>
c = 3a
c + a = 48
c = children, a = adults
<em>In the second sentence (in bold), we can replace c with 3a</em>
c + a = 48 → 3a + a = 48
<em>Simplify</em>
3a + a = 48 → 4a = 48 → a = 12 (divide both sides by 4)
<em>We can then substitute or plug in a = 12 into the original equation:</em>
c + a = 48 → c + 12 = 48 → c = 36 (subtract 12 from both sides)
<h3>There are 36 children</h3>