I am really lost with this.

Calculate the amount of energy released in kJ/mol for the fusion reaction of two 1H atoms to yield a 2H atom and electron:
1/1H+1/1H→2/1H+0/1e
The atomic masses are
0/1e (0.0005486 u),
1H (1.00783 u),
2H (2.01410 u),
and 3He (3.01603 u).

I know you have to subtract the product amu from the reactant amu to find the mass change before using E=mc^2

I do not know why they gave me the atomic mass of 3He or how to use E=mc^2. If someone could show me step by step that would be great but know its a long problem so I'd appreciate any help you can give.