Have you ever heard to use parentheses?
(5V - 10)/21 // (V - 2)/35
(5V - 10)/21 * 35/(V - 2)
35(5V - 10)/21(V - 2)
(175V - 350)/(21V - 42)
------these are not factors, so I see no further simplification
(5V - 10)/21 // (V - 2)/35
(5V - 10)/21 * 35/(V - 2)
35(5V - 10)/21(V - 2)
(175V - 350)/(21V - 42)
------these are not factors, so I see no further simplification