Using a calculator, you should find that
-2 & 4/5 = -(2 + 4/5) = -(2+0.8) = -2.8
-31/5 = -6.2
So the unknown number is between -6.2 and -2.8
Possible whole number answers could be: -6, -5, -4, -3, or -2
If your teacher allows decimal answers, then there are infinitely many possible ways to answer.