How do you evaluate variable expressions?

To evaluate a variable expression means to find the value of the expression when the variable is (or variables are) replaced by a number (or numbers).

Here is a variable expression: #3x+7#.

To evaluate this expression when #x = 2# mean to substitute #2# in place of #x#, and find the number (the value). [see note below]

#3(2)+7 = 6+7 = 13#.

When #x=2#, the expression has a value of `#13#.

If we evaluate the same expression when #x=10#, we get:

#3(10)+7 = 30+7 =37#.

Here's a variable expression with two variables : #3x-2y#

In order to evaluate this expression, we'll need values (numbers) for both #x# and #y#.

Evaluate the expression #3x-2y# when #x=4# and #y=5#

#3(4)-2(5) = 12 - 10 = 2#

If we change the values to #x=5# and #y=4#, we get
#3(5)-2(4) = 15 - 8 =7#.

Note:

To avoid saying exactly the same thing over and over, we use the following to mean the same thing:

evaluate the expression when #x = 2#
evaluate the expression for #x = 2#
evaluate the expression at #x = 2#
evaluate the expression assuming #x = 2#
evaluate the expression if #x = 2#

Also

If there is only one variable in an expression, we don't always need to mention it:
evaluate the expression for #2#
evaluate the expression at #2#