Start by assigning a variable name to the "number" -- call it x.
Now write out what they say:
"The product of 1/3 and the sum of a number and 3" -- that's 1/3 times x + 3, or (1/3)(x + 3) or (x + 3) / 3
It's "between negative one and five"
So that means:
-1 < (x + 3) / 3 < 5
So now solve this inequality. First multiply everything by 3.
-3 < x + 3 < 15
Subtract 3 from all the terms:
-6 < x < 12
So the answer is, all numbers between -6 and 12