Question

(21+13)+..........= 21+(13+18)

Answer 1

=18
21+13+18=52
21+13=34
52-34=18

Answer 2

... = 18

Note the equal sign. The end result must be the same

for 21 + 13 + x to equal 21 + 13 + 18, x must be 18

18 is your answer

hope this helps