3( 1+2) means 3 times the number in brackets. There is a law in mathematics that if there are parentheses, the number in parentheses is calculated first, so the number in parentheses is1+2 = 3; Then multiply it by 3,3× 3 = 9 before the brackets.

In arithmetic

Addition has several important properties. It is interchangeable, which means that the order is not important, it is interrelated, which means that when more than two numbers are added, the order in which the addition is performed is not important. Repeatedly adding 1 is the same as counting; Adding 0 will not change the result. Addition also follows related operations such as subtraction and multiplication.