The curve must have this orientation, since we know it has horizontal axis and passes through `(2, -1)`:

parabola with horizontal axis

So we need to use the general formula

`y^2= 4px`

We need to find `p`. We know the curve goes through `(2, -1)`, so we substitute:

`(-1)^2= 4(p)(2) `

→ `1 = 8p`

→ `p = 1/8`.

So the required equation is `y^2=x/2`.