15 + (-2)
indicates that you need to perform an addition of two numbers:
one is 15, and the other number is a negative one: -2
so what you do when facing such (a parenthesis) following the plus symbol, is to simply remove the parenthesis and convert that plus into a minus (using the convention: "positive times negative gives a negative answer")
So simply do:
15 - 2 = 13
In summary: adding a negative number is like subtracting the number
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