Yes, (8 + 12) / 2 looks right. You don't need to memorize rules to decide this.
An arithmetic expression is a syntactically correct combination of numbers, operators, parenthesis, and variables.
You have not (officially) seen variables yet, so ignore that part of the definition. Here are several arithmetic expressions, similar to those you saw in the previous chapter:
25 + 15 32.128 - 19.6 + 3.2 -14 / 3 1.243 ^ 5 -23.77 * -2 10 - 5 - 8
Remember that syntax means the rules for putting together a correctly formed statement. Arithmetic expressions are parts of statements, so must follow syntax rules in order to be correct. You are already familiar with these rules: they are the same rules as for pencil-and-paper arithmetic.
Is the following an arithmetic expression?
89.3 + / 2