When the slope is zero, the line is horizontal. Its equation will be of the form
... y = constant
The y-value of your point is specified as -9, so the equation of the line through that point is
... y = -9
What are the three question?
ANSWER

EXPLANATION
We want to find the number of years that it will take the population to double.
To do this, we have to apply the exponential growth function:

where y = final value
a = initial value
r = rate of growth
t = time (in years)
For the population to double, it means that the final value must be 2 times the initial value:

Substitute the given values into the function above:

To solve further, convert the function from an exponential function to a logarithmic function as follows:

Solve for t:

It will take 9 years for the population to double.
Let shirts be x and ties be y
3x+5y=$60 Equation 1 (Equation 1 will be denoted by *)
2x+3y=$39 Equation 2 (Equation 2 will be denoted by !)
*x2 6x + 10y = $120
!x3 6x + 9y = $117
*-! y = 120-117 =3
Ties are $3 each
Substitute this value back into one of the original 2 equations:
3x+5y=$60
3x+(5x3) = 60
3x+15=60
3x=45
x=15
Shirts are $15