The way to install these PHP extensions depends on which OS you’re using.

For Ubuntu 14.04 (you only need to install the curl extension as mbstring should be built in):

sudo apt-get install php5-curl

For Ubuntu 16.04:

sudo apt-get install php7.0-curl
sudo apt-get install php7.0-mbstring

For Centos 7 (you only need to install the mbstring extension as curl should be built in):

yum install php-mbstring

Alternatively, to avoid problems related to missing PHP extensions, you could manage your server with ServerPilot which installs all common PHP extensions. The full list is here:

https://serverpilot.io/community/articles/available-php-extensions.html