In your equation: a+(-4)=-13
you then add four to each side,
so a= negative 13 plus 4
so a= -9
The first thing we must do for this case is to define variables.
We have then:
x: number of slices
y: total cost
We write the linear function that relates the variables.
We have then:

Then, we evaluate the number of slices to find the total cost.
-two slices cost:
We substitute x = 2 in the given equation:

Answer:
two slices = 2.2 $
-ten slices cost:
We substitute x = 10 in the given equation:

Answer:
ten slices = 11 $
-half a slice cost:
We substitute x = 1/2 in the given equation:

Answer:
half a slice = 0.55 $
Answer: I believe its 24 meters.
Explanation: Just divide 120 by 5