if x+ a= b/3x and b does not = 3 then x =?

Question

i cant seem to find a way to seprate the two x's since 3/b is always attached.

subtract (b/3)x from both sides

x- (b/3) x+a=0
x(1-b/3)= -a

x= a/(1-b/3)

i looked at the back of my book and the answer is 3a/b-3