Rule1: Always start with the inner most bracket first

-5-(11-(13+5))

13+5 = 18 so put 18 instead of that in the expression

-5-(11-18)

Now the innermost bracket is (11-18)

-5-(11-18)

Now we calculate 11-18, which is equal to -7 hence we put -7 instead of that,

-5-(-7)

here is the important part which most people miss. When we subtract a negative value from a number its the same as adding the positive value. we are subtracting -7(the negative value) which is the same as adding the positive value to it. Hence we can get to the below expression