If an expression doesn't involve a variable, evaluating it simply means to simplify it, following PEMDAS rule. For example, if we start with
![2+3\cdot 5+(3+7)\cdot 2](https://img.qammunity.org/2020/formulas/mathematics/middle-school/3y4znek8r7qo8heelwujked89zaswomnzq.png)
We perform parenthesis first:
![2+3\cdot 5+10\cdot 2](https://img.qammunity.org/2020/formulas/mathematics/middle-school/2tg9e7lz8m6sscbpw1o29hg2ybqc1g13fp.png)
Then multiplications:
![2+15+20](https://img.qammunity.org/2020/formulas/mathematics/middle-school/qv0jp4zxuiy4ihtw36tr5vwdzms95ffvm7.png)
And finally additions:
![37](https://img.qammunity.org/2020/formulas/mathematics/middle-school/nv3lpkhvtm7hqxgmyh9dd26i3ncsw7p4qq.png)
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
![f(x,y)=3x+2xy](https://img.qammunity.org/2020/formulas/mathematics/middle-school/pmp28rbs8plwaggmdodhti1xdbz5wge16j.png)
If we choose x=4 and y=7 the expression becomes
![f(4,7)=3\cdot 4+2\cdot 4\cdot 7=12+56=68](https://img.qammunity.org/2020/formulas/mathematics/middle-school/4q0vyql5447ik8utjft2vl7yvrx6kuzuon.png)