[Note that you don't have to evaluate k to solve the problem]
[tex]x = \frac{kuv}{(u+v)}[/tex]
That means that
[tex]x \frac{(u+v)}{(uv)} = k, a \: constant[/tex]
So, you want [tex]x[/tex]such that
[tex]x \frac{(3+3)}{(3 \times 3)} = 3 \frac{(3+1)}{(3 \times 1)}[/tex]
[tex]x=6[/tex]
