Question

How do you graph `r = 2`?

Answer 1

You can change it to rectangular coordinates or consider that is the graph produced by a point at distance `2` from the origin and at all the possible angles `theta` from the positive `x` axis (and so describing a circle!).

Explanation:

Consider the following diagram:

We can see that the relationships between rectangular and polar coordinates are:
`r=sqrt(x^2+y^2)`
`theta=arctan(y/x)`
and:
`x=rcos(theta)`
`y=rsin(theta)`

In your case you have:
`r=2`
Change it into rectangular to get:
`sqrt(x^2+y^2)=2`
Square both sides:
`x^2+y^2=2^2`
Which is the equation of a circle of radius `2` and centred in the origin.