What is a variable?
A variable is a letter that represents an unknown number.
eg. X, A, N.
What is an expression?
An expression is any single number or variable or combination of operations ( Subtraction, addition, multiplication and division) involving numbers and variables.
eg. 5, n, 8t, x+1, 2y-7.
Here is a link to help you if you are still having troubles with variables and expressions: