The short answer is,
ssh srv_1 'ssh srv_2 "command"'
where command is whatever command you want to run.
It also works without the single quotes, but I like them to aid in understanding.
ssh srv_1 ssh srv_2 "command"
Using the quotes gives us some structure to work within, but as you can see, they're not fundamentally necessary.
This also works,
ssh srv_1 ssh srv_2 command
We can't tell from the information you've given us if this will work, or if the command will do anything, but the above syntax is the approach I would take.
If you have to start using variables or command substitution then it'll get more complex, more quickly and you'll need to be careful with quoting and escaping.
Here's an example of it working.
Neptune is my host machine.
Trinity is another server.
Matrix is the final server.
tony@neptune:~$ ssh trinity 'ssh matrix "hostname"'
So the command returns the hostname matrix, because it has executed on the server matrix.
NB: Changing a directory in this way will have absolutely no effect, directory changes are relevant only to the shell in which they're executed.
Here's what I mean,
tony@Neptune:~$ ssh trinity 'ssh matrix "cd /tmp"'
tony@Neptune:~$ ssh trinity 'ssh matrix "pwd"'
The second command shows the previous
cd command has had no lasting effect.