Is true that you have to have to add before you subtract

I dont think its true. If your statement is true, there must be a parenthesis.

Between addition and subtraction, there is no reason why either one

needs to be done before the other one. In each problem that you may

be solving, you can just decide which one would be easier or more

convenient to do first, and then go right ahead and do it. The choice

is entirely up to you.