Somehow I can't get it for this basic bash problem. maybe someone can help.

What I try to do is:

a="world"
b="hello"

how can I move this into $c so that I can replace "helloworld" with "world hello" in sed like:

sed "s/\helloworld/ ${c}...

I tried several combinations but all with a wrong result:

c="${a};${b}"

echo $c;
hellod