The question is accompanied of a table showing the atomic masses and abundances of the five isotopes and asks to calculate the atomic mass of the element with five significant figures. You just have to add the products of the atomic mass of each isotope times its abundance and divide by 100. This is the calculation:
(179.946706*0.12 + 181.948206*26.5 + 182.9502245*14.31 + 183.9509326*30.64 + 185.954362*28.43 ) / 100 = 183.84 (which is rounded to five significant figures)
