Algebra can consist of any combinations of numbers, variables, operators and exponents. If we know how to interpret the expression we can "evaluate" it.
Evaluating an expression means to find its value (what it's equal to) if we know what each variable represents.
Let's just consider the expression:
3x + 7
This expression says to multiply 3 and some mystery value "x", then add 7 (we must follow the order of operations! See the early blog post to review the order of operations). This is a pretty easy expression to evaluate.
Let's find the value of the expression if we let x = 4.
When x = 4, we can rewrite the statement and instead of writing the "x" we can replace it with 4.
So the expression would become:
3(4) + 7
Note the use of parentheses to remind us that the expression was 3 times x, or now, 3 times 4.
The value of this expression is now 12 +7, which is 19.
Evaluating expressions becomes trickier when the expressions themselves have more terms, involve exponents, or tricky orders of operations.
For some basic review, check out Khan Academy's site (click here for video) and (click here for practice problems).
For some fancy, challenging problems, check out this website (click here). The answers are provided, so you can check your work.
This site walks you through the evaluation process step-by-step. (click here) If you go to this link (click here) you can practice some really challenging problems. They call it a "work out", and it will wear your brain out. But it's like any other muscle... the more you exercise it, the stronger it will become. Try to get 8 right in a row before you move on to a different topic.