12 + 10 ÷ 2 − 3 is an "order of operations" problem. "PEMDAS" refers to the order in which mathmatical operations must be carried out: parentheses, exponentiation, mult/div, add'n/subtr.
In 12 + 10 ÷ 2 − 3 you MUST divide 10 by 2 before you do anything else.
This gives you 12 + 5 − 3. Since + and - are on the same level of operations, you can work from left to right: 12+5 is 17; from 17 we subtract 3, arriving at 14 (answer).
