tolong ya dengan cara

7/(7+5) = 9/(9+a)
7/12 = 9/(9+a)
9+a = 12/7 x 9
9+a = 108/7
(9+a) x 7 = 108
63 + 7a = 108
7a = 108 - 63
7a = 45
a = 45/7
a = 6,4285.. atau 6,42 ( B )