Tour of Go exercise #18: Slices

I am trying to complete the Exercise: Slices from the Go Tour.
However I don't really understand what I'm being asked to do.

Implement Pic. It should return a slice of length dy, each element of which is a slice of dx 8-bit unsigned integers. When you run the program, it will display your picture, interpreting the integers as grayscale (well, bluescale) values.

I have the following code

package main

import "golang.org/x/tour/pic"

func Pic(dx, dy int) [][]uint8 {
    a := make([][]uint8, dy)
    return a
}

func main() {
    pic.Show(Pic)
}

I have created the slice of length dy so far. However I don't understand the next step. Do I need to create a for loop in which I assign every element of my slice to a value in the range of dx? I don't ask for code but rather for an explanation/clarification

Do I need to create a for loop in which I assign every element of my slice to a value in the range of dx?

Yes:

• an outer loop to assign a[x] with a []uint8 slice of dx size,
• with an inner loop 'y' for each element a[x] (which is a []uint8) in order to assign a[x][y] with the asked value (ie, one of the "interesting functions" like x^y, (x+y)/2, and x*y).
  x and y are indexes in a range over a slice (a, then a[x]): see "For statements".

I like (x ^ y) * (x ^ y):

As rwilson04 comments below:

x*x + y*y is another good one

Waqas Ilyas suggests in the comments: y * 10000 / (x + 1)

EdvardM suggests in the comments: (x+y)/2 * (x^y):

Chubby Cows proposes in the comments: ((x - y) * -1) * (x + y) / 2: