Normal sudoku rules apply. The two lines are equal sum lines ie The sum of the digits on a line within a particular 3x3 region must be the same for all the regions the line passes through. Different lines may have different sums. Each letter represents a unique digit. If a letter is in the grid, that digit must be put in the same cell. Clues outside of the grid are x-sums clues ie they indicate the sum of the digits of the first x cells in its row or column, where x is the digit placed in the first cell in that direction.