If a/b = c/d, apply a * d = b * c:
a = b - 3a
b = 9
c = a
d = 3
3(b - 3a) = 9a
Expand:
3b - 9a = 9a
Subtract 3b from both sides:
3b - 9a - 3b = 9a - 3b
Simplify:
-9a = 9a - 3b
-9a - 9a = -3b
-18a = -3b
Divide both sides by -18:
-18a/-18 = -3b/-18
Simplify:
a = b/6
b = 6a
