The x value given is -1.
On the coordinate-plane, a coordinate is labeled by (x, y).
That's how we get x = -1.
We plug in -1 for x, or, just substituting -1 for x.
f(x)=-2x+3
↓
f(-1)=(-2)(-1)+3
↓
f(-1)=2+3
↓
f(-1)=5
The value 5, for "y," would complete the ordered pair.
