If an expression doesn't involve a variable, evaluating it simply means to simplify it, following PEMDAS rule. For example, if we start with

We perform parenthesis first:

Then multiplications:

And finally additions:

If, instead, an expression involves variables, evaluating the expression means to plug a specific value for the variable(s): for example, if we start with

If we choose x=4 and y=7 the expression becomes
