Answer
1H-1H: 2.01566 amu
1H-2H: 3.02193 amu
2H-2H: 4.02820 amu
Work Step by Step
You know that there are three different peaks. You calculate each by using the given atomic masses of the different isotopes. You go through each possible combination, adding together the atomic masses of the isotopes the molecule is made out of. For example, 1H-1H has two 1H atoms, so the atomic mass will be 1.00783 amu (the given atomic mass for 1H) times 2 which is equal to 2.01566 amu.
