In essence, you are finding the ratio of oxygen mass to total mass.
The first step in solving this problem is to determine the molar mass of the entire substance:
Ca = 40.08
N = 14 --> multiply this by 2 because the subscript outside the brackets applies to the entire thing.
O = 16 --> multiply this by 2 (the 2 beside the O indicates this), then multiply by 2 again, indicated by the two outside the brackets.
Total Mass = 40.08 + 2(14) + 4(16)
Total Mass = 132.08
Now, you set up the ratio of ONLY oxygen to the total mass and find the answer:
Now, take this decimal and multiply it by 100% to turn it into percent form: 0.4845 x 100% = 48.45%