Normal sudoku rules apply. Digits on the blue line have an equal sum, N, within each 3x3 box through which it passes. If the line passes through the same box more than once, each line segment sums to N separately. There is exactly one 'doubler' in each row, column and 3x3 box. These are the digits 1-9 once each. For the purpose of the equal sums line, a digit in a 'doubler' cell counts as twice the digit's value. The yellow line between r6c5 and r6c6 is purely aesthetic, i.e. box 5 has two distinct line segments.