If x is made twice as large, then also y becomes twice as large.
This is true for graph 1 only, because:
on graph 1 there are points (2, 5) and (4, 10) to be found.
Multiplying x : 2 by 2 gives 4 and
multiplying y : 5 by 2 gives 10.
In the next slide we come back at this with the Theory from the book.