Write as an algebraic expression, inequality, or equation:

the quotient of 18 and a number

18/x

this would be how you write it as an algebraic expression

This would be an expression because it does not give any other "answer" or "equals to".

The "quotient" means that we divide. Since 18 is written first, we can assume it is the dividend, and x is the divisor:

18/x