Sure, if you want to use Apache to handle distributing your frontend and handling the WP backend, read up on Virtual Hosts: https://www.digitalocean.com/community/tutorials/how-to-set-up-apache-virtual-hosts-on-ubuntu-14-04-lts
The gist of it is that you'll have a VH for your frontend
<VirtualHost *:80>
ServerAdmin [email protected]
ServerName example.com
ServerAlias www.example.com <-- see here
DocumentRoot /var/www/example.com/client
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>
and a VH for your backend
<VirtualHost *:80>
ServerAdmin [email protected]
ServerName example.com
ServerAlias api.example.com <-- and here
DocumentRoot /var/www/example.com/server
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>