Hi Jamal,
You can use conv and deconv for solving this equation. You will be getting N length vector only as an output. Below code might be of use. This will be only valid if conv(a,b) = conv(x,c) has a valid solution x. otherwise there will be some remainder instead of ~.
% Let a,b,c be N length vector
[x,~] = deconv(conv(a,b),c);