Answer:
Part a) 
Part b) The expression that represent the cost of fencing the field is 
Part c) 
Step-by-step explanation:
<u>The complete question in the attached figure</u>
Part a) we know that
The perimeter of the rectangular field (excluding the width of the gate) is equal to

we have

substitute the given values


Part b) we know that
To find out the cost of fencing the field (excluding the gate) , multiply the perimeter by the cost of $28 per yard
so

Part c) we have
h=5
substitute the value of h in the expression of the cost

f(x) = 2
-4x
Step-by-step explanation:
Step 1 :
Given, f(x) = a(x - h)2 + k
Point on the parabola is (3, 6)
Vertex (h,k) = (1,-2)
Step 2:
Substituting the vertex in the equation we have,
f(x) = a(x-1)2 -2
Substituting the point (3,6) in this we have,
6 = a(3-1)2 - 2 => 6 = 4a -2
=> 4a = 8 => a = 2
Step 3 :
Substituting the value for a and the vertex in the given equation we have
f(x) = 2(x-1)2 -2 = 2(x2 - 2x + 1) -2 = 2x2 - 4x
=> f(x) = 2
-4x which is the standard form