How to Find the Mass of an Individual Element in a Compound/Molecule
Step 1: Find the Molar Mass of Adrenaline 

To do this, you must need to know its chemical formula which is $\ce{C9H13NO3}$. Now to calculate its molar mass, we add up all of the molar masses of each atom: $$M =9\times12~\mathrm{g~mol^{-1}} + 1\times13~\mathrm{g~mol^{-1}} + 14~\mathrm{g~mol^{-1}} + 3\times16~\mathrm{g~mol^{-1}} = 183~\mathrm{g~mol^{-1}}$$

Step 2: Find the Mass of Oxygen in 1 mol of Adrenaline

In each molecule of adrenaline, there is 3 molecules of oxygen atoms. This means that in $1\ \mathrm{mol}$ of adrenaline, there are $3\ \mathrm{mol}$ of oxygen. Therefore the mass of oxygen in $1\ \mathrm{mol}$ of adrenaline is: $\mathrm{3~mol\times16~g~mol^{-1} = 48~g}$. 

Step 3: Calculate Percentage Composition of Oxygen by Mass

We now know that in every mole of adrenaline, there are $48\ \mathrm{g}$ of oxygen. We also know that $1\ \mathrm{mol}$ of adrenaline weighs $183\ \mathrm{g}$. Therefore in $183\ \mathrm{g}$ of adrenaline, there are $48\ \mathrm{g}$ of oxygen. All have to do is simply divide by them by each other and multiply by $100$ to get the percentage composition of oxygen. Therefore the percentage composition is $26.23\,\%$.

Step 4: Calculate Mass of Oxygen in Sample

Now we know the percentage composition of oxygen which is $26.23\,\%$. This means that in every gram of adrenaline, there is $0.2623\ \mathrm{g}$ of oxygen. Therefore to find the mass of oxygen in the sample: $$m_\text{oxygen} = 0.2623 \times 6.8~\mathrm{g} = 1.78~\mathrm{g}$$