Why were so many cities able to develop between the Tigris and Euphrates rivers? The cities were protected by mountains to the west.
The forests provided plentiful lumber for construction.
The cities were protected by a vast desert to the east.
The rivers were used for irrigation, which led to a food surplus.

What exactly do you mean 

Amath expression is a collection of one or more terms, which are separated by the signs ( + , - , x , / ).example :   8+b8+b  or  a−3a−3in algebra, an equation is a mathematical statement with two expressions where, each expression is separated by an equal (=) sign. variables can be present on both the sides of the equality (=) sign and we use letters to denote such variables.example 1:   x=4+2x=4+2  , [here, we can observe that the variable is present only on one side of the equation].example 2:   2x+3=x+42x+3=x+4  , [here, we can observe that the variables are present on both side of the equation].