The answer is:
(y - 7)(y + 4)
Work/explanation:
We need to think of two numbers whose product is -28 and whose sum is -3.
These numbers are -7 and 4.
Now, remember that a factored expression looks like this:

What goes in the blanks are the numbers -7 and 4:

Simplify

Hence, the answer is (y - 7)(y + 4).