Hello!
B + 21 = 17
B = -4
To get your answer, the only number available must be a negative number, as 21 is already bigger than 17.
Simply by adding 4 to 17 you get 21, so then make that a negative
-4 added to 21 makes a postive number which would be 17
-4 + 21 = 17
Hope this helps!
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