If an expression doesn't involve a variable, evaluating it simply means to simplify it, following PEMDAS rule. For example, if we start with
[tex]2+3\cdot 5+(3+7)\cdot 2[/tex]
We perform parenthesis first:
[tex]2+3\cdot 5+10\cdot 2[/tex]
Then multiplications:
[tex]2+15+20[/tex]
And finally additions:
[tex]37[/tex]
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
[tex]f(x,y)=3x+2xy[/tex]
If we choose x=4 and y=7 the expression becomes
[tex]f(4,7)=3\cdot 4+2\cdot 4\cdot 7=12+56=68[/tex]
