You can
put this solution on YOUR website! .
(1) 2d + 4 = 5 mod 7 =====>
(2) 2d = 5 - 4 mod 7
(3) 2d = 1 mod 7
(4) d = 4 mod 7 ANSWER
In this chain of equations mod 7, only transition from (3) to (4) requires explanation.
But 2d = 1 mod 7 is the same as 2d = 8 mod 7, which makes this transition OBVIOUS.
Solved.
