I need to transpose this formula to make "b" the subject. It looks very simple but is giving me a headache.

a=b/3+b

I think the word "transpose" is incorrect.

You want to solve it for "b".

a = b/3 + b
a = b/3 + 3b/3 (find the common denominator)
a = 4b/3
3/4 * a = b
b = 3a/4
viola! It was very simple.

a = b/(b+3)
a(b+3) = b
ab + 3a = b
3a = b - ab = b(1-a)

3/(1-a) = b