Answer:
At-207 (meaning At with the number on the top left being 207 and the number at the bottom left being 85.
Explanation:
The sum of the mass numbers has to equal on both sides, so we need 211 = 4+a
a=207
The sum of the atomic numbers as to equal on both sides, so 87=2+b
b=85
The element is determined by the mass number.
