The relative abundance of silicon as per the mass spectrum is
92.21% of 27.97 amu, 4.79% of 28.97 amu and 3.09% of 29.97 amu.
The average atomic weight is calculated as the sum of products of relative abundance of various isotopes and their atomic mass.
i.e. Average atomic mass = 0.9221 x 27.97 + 0.0479 x 28.97+ 0.0309 x 29.97
= 28.11 amu
Thus the analysis of mass spectrum reveals that the silicon atom has an average atomic mass of 28.11 amu.
Hope this helps.