I want to design a context free grammar for the following
Since this seems very much like homework, i will give you a little Hint, taking as example a sub-case of your language: 

The GFC for the string $a^3b^3$ is the following:

$S ⇒ aSb$

$aSb ⇒ aaSbb$

$aaSbb ⇒ aaaSbbb$

$aaaSbbb ⇒ aaabbb$

You can take the above as a starting point from which to adapt the grammar to your case.