Let r = 9 and solve for a.
a + 3 = (2a + 7)/r
a + 3 = (2a + 7)/9
9(a + 3) = 2a + 7
9a + 27 = 2a + 7
9a - 2a = 7 - 27
7a = -20
a = -20/7
I believe this is what you seek.
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