Question 141879
I don't have this text available to me, however, I suspect I know what they are asking. Note that most tutors here will not have the text available to them.

In a function, we essentially are taking some value for one variable to produce another. There is no possible way in the x-y coordinate system for a function f(x)=(anything involving x)to produce two function values. Moreover, for any number there is only one possible outcome for an expression.

Consider:
f(x)=1/x

f(1)=1  always
f(2)=1/2  always
f(3)=1/3 always
.
.
.
etc.