**For Company Y:**

let #c_1=# the cost of making a phone call with Company Y's plan.

#color(red)(c_1=10+0.1x)#

**For Company Z:**

let #c_2=# the cost of making a phone call with Company Z's plan.

#color(blue)(c_2=0.2x)#

**For the difference between the two companies:**

It could go both ways:

#color(blue)(c_2)-color(red)(c_1)=0.2x-(10+0.1x)=0.2x-10-0.1x=color(green)(0.1x-10)#

#color(red)(c_1)-color(blue)(c_2)=(10+0.1x)-0.2x=10+0.1x-0.2x=10-0.1x=-color(green)(0.1x-10)#

As you can see, both paths lead to the same answer.

If there were a function #f(x)# that represents the difference in cost between Company Y and Company Z, then

#f(x)=-0.1x-10#

Hope this helped!