X^2-4x-5=0
according viet’s theorem
the roots are 5 and -1

The average of 45 and 19 is
32.
When x=-1:

Ok that gives us a little more information.
If we implicitly differentiate with respect to t, from the very start, then we can apply our product rule, ya?

The right side is zero, derivative of a constant is zero.
Where x' is dx/dt and y' is dy/dt.
From here, plug in all the stuff you know:
y' = -3
x = -1
y = 4
and solve for x'.
Hope that helps!
First we want to find how much it costs for 1 stamp, so we do 8.28 divided by 18, which equals 0.46. Then, we multiply 0.46 and 12, which is 5.52. Therefore, it would cost $5.52 to buy 12 stamps.