Step-by-step explanation:

Answer: X = 2
Step-by-step explanation: The correct answer to this question is that x=2. Regardless of what point you go to on the line, it always has a value of 2 because it is going vertically (up and down) at 2.
A. The term "units" refers to the measurements in the original question or problem. When looking at the question, the units we are measuring are dollars/feet.
B. In this case, the f'(x) means we are looking for the cost (in dollars) to drill the next foot down.
C. Since the cost increases the more feet we drill down, the sign of f'(x) is positive.
90 = 30+10+3x
This equation shows that all the prices of the the bag, eraser, and pencils equals $90. So now all we have to do to find the price of a pencil is solve the equation.
First add the 30 and 10 together.
90 = 40+3x
Then subtract 40 from each side
50 = 3x
Lastly divide each side by 3
x = 16.67
The price per pencil is $16.67
Hope this helps!