An ordered pair is a pair of values called the input and output, or just the x and the y on your graph. The input is your x and the output is your y.
When you're given a function, as long as you have a value for your x, you can always easily determine your value of y. Hence, you input -1 and whatever the result is, you have your y value.
The function is given as
f(x) = -2xof y