First, find the total mass.
34.5 g + 18.2 g + 2.6 g = 55.3 g
Divide the mass of each element by total mass.
34.5 g / 55.3 g = .62 x 100 = 62% element a
18.2 g / 55.3 = .33 x 100 = 33% element b
2.6 g / 55.3 = .047 x 100 = 4.7% element c
Hope I helped!