Answer
See work below.
Work Step by Step
According to the picture, there are 24 atoms, 8 of which are blue and 16 of which are orange. Using isotopic masses in the problem, the atomic mass of element X is -
$\frac{8}{24}$ (47.621 amu) + $\frac{16}{24}$ (51.217 amu)= 15.8737+34.1447=50.0184=50.018 amu