what is the mass of chromium in this sample?
I like to solve such assignments using Python as a scientific calculator.
import scipy.constants 

# number of atoms in sample
N_ATOMS_TOTAL = 2.13E+24 

# atomic mass of chromium
M_CR = 51.9961 # g/mol

# ratio of chromium atoms to total atoms in K2Cr2O7
CR_RATIO = 2/11

# moles of Cr in sample
n_cr = N_ATOMS_TOTAL * CR_RATIO / scipy.constants.Avogadro

mass_cr = n_cr * M_CR # mass of chromium in g

mass_cr

Let me explain:

You have an unknown mass (in gramm) of potassium dichromate, $\ce{K2Cr2O7}$.

The total number of atoms in this sample is $2.13\cdot10^{24}$.


How can you calculate the number of chromium atoms in the sample?

You need to determine the ratio of $\ce{Cr}$ to all atoms in $\ce{K2Cr2O7}$, and that's $\frac{2}{11}$ atoms.

I know the number of $\ce{Cr}$ atoms now, but how many moles is that?

You know that 1 mol of an element has $6.02214129\cdot10^{23}$ atoms of that element (Avogadro's number).
Divide the number of $\ce{Cr}$ atoms determined before by Avogadro's number to determine the number of moles of $\ce{Cr}$.

I know the number of moles of $\ce{Cr}$ now, but what is the weight?

You know that the standard atomic weight of $\ce{Cr}$ is 51.9961.
Multiply  the standard atomic weight with the number of moles to get the weight.