389.88094 amu
First we look up the atomic mass of all elements contained in calcium iodate using the periodic table:
Ca: 40.078
I: 126.90447
O: 15.999
As an intermediate step we calculate the molecular mass of the ion IO3:
126.90447 + 3*15.999 = 174.90147
Then we calculate the mass of one calcium atom and 2 iodate ions:
2*174.90147 + 40.078 = 389.88094 amu
