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!