Here's how you can approach such problems.
The question asks you determine what the mass of a large group of iron atoms,
Now, they tell you that each of those atoms has a mass of 56 u. The symbol "u" is used to denote the equivalent of
In more practical terms,
Since each atom of iron weighs 56 times more than that, you have
Each iron atom has a mass of
An interesting thing to notice here is that if you use the number of atoms present in 1 mole of iron, and multiply this number by the mass of an individual iron atom, you'll get the molar mass of iron.
By definition, one mole of any substance contains exactly
In your case, one mole of iron would have a mass of
If you take into account rounding and sig figs, this result matches the actual molar mass of iron, which is 55.845 g.
So remember, one "u" is always equal to