There are 9 rows of numbers across and 9 columns of numbers down.
Numbers 1 through 9 need to go across and down.
You can only use each number once in each row, column, or 3×3 square.

l 6 l _ l 3 ll _ l _ l 1 ll _ l 4 l _ l
l _ l _ l 8 ll _ l _ l _ ll _ l 5 l 9 l
l 9 l _ l _ ll _ l 7 l 4 ll _ l _ l _ l

l _ l 7 l _ ll _ l 2 l _ ll 9 l _ l 3 l
l _ l _ l 1 ll 6 l _ l 5 ll 7 l _ l _ l
l 8 l _ l 2 ll _ l 3 l _ ll _ l 1 l _ l

l _ l _ l _ ll 2 l 9 l _ ll _ l _ l 1 l
l 1 l 3 l _ ll _ l _ l _ ll 6 l _ l _ l
l _ l 5 l _ ll 4 l _ l _ ll 3 l _ l 8 l