atomic weight = massa x fracta + massb x fractb
If there were three isotopes, you would add a 'c' entry. If there were four isotopes, you'd add a 'd', etc.
If chlorine has two naturally-occurring isotopes where:
Cl-35 mass is 34.968852 and fract is 0.7577
Cl-37 mass is 36.965303 and fract is 0.2423
atomic weight = massa x fracta + massb x fracb
atomic weight = 34.968852 x 0.7577 + 36.965303 x 0.2423
atomic weight = 26.496 amu + 8.9566 amu
atomic weight = 35.45 amu
The sum of the fractional abundance values must equal 1.
Be sure to use the mass or weight of each isotope and not its mass number.