We must find the common denominator.
It's LCM of 9 and 6.
Multiples of 6: 0, 6, 12, 18, 24, ...
Multiples of 9: 0, 9, 18, 27, ...
LCM(6, 9) = 18
18 = 3 · 6
18 = 2 · 9

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Other method:
