Answer
The element is silicon Si . The the atomic mass that we found 28.0857 amu $\approx$ 28.0855 amu. (This last number is atomic mass of Si in periodic table of elements.)
Work Step by Step
The atomic mass of the element is found by multiplying each isotopic mass by its fraction, and then add all of them.
27.977 amu x 0.9221 = 25.7976 amu
28.976 amu x 0.047 = 1.3619 amu
29.974 amu x 0.0309 = 0.9262 amu
____________________________
Sum: 28.0857 amu
We see in the periodic table of elements that silicon Si , has the atomic mass 28.0855 amu, that is close to the number we found (within the experiment error) .
Therefore the element is silicon Si . The the atomic mass that we found 28.0857 amu $\approx$28.0855 amu.
