Answer
Atomic mass = 121.8 amu
The element is Antimony (Sb)
Work Step by Step
Mass of isotope 1 = 120.9038 amu = $m_{1}$
Relative abundance of isotope 1 = 57.4% = 0.574 = $x_{1}$
Mass of isotope 2 = 122.9042 amu = $m_{2}$
Relative abundance of isotope 2 = 42.6% = 0.426 = $x_{2}$
Atomic mass = $m_{1}$$x_{1} + m_{2}$$x_{2}= 120.9038 \times 0.574 + 122.9042 \times 0.426 = 69.3987 + 52.3572 = 121.8$
The element is Antimony (Sb)
