The side of a regular decagon is $l$. Express the perimeter of the decagon using $l$.

Check whether the following expression is with numbers only:

$2z-13$

Check whether the following expression is with numbers only:

$5(31-6)-6\times 3+11$

Check whether the following expression is with numbers only:

$7-11+13$

Check whether the following expression is with numbers only:

$5+5x$

Check whether the following expression is with numbers only:

$30-3x$

Identify the operation (addition, subtraction, division, multiplication) in forming the following expression and explain how it is formed:

$x+4$

Identify the operations (addition, subtraction, division, multiplication) in forming the following expression and explain how it is formed:

$7x-3$