How many particles are there in 0.35 mol of any substance? …

Step 1. (a) $0.35 \times 6.022 \times 10^{23} = 2.11 \times 10^{23}$.

Step 2. (b) These are molecules of $\text{O}_2$, each with $2$ atoms: $2 \times 2.11 \times 10^{23} = 4.22 \times 10^{23}$.