First, we want to convert atoms of hydrogen to moles of hydrogen

2.84 x 10²³ atoms H

(1 mole H) / (6.022 x 10²³ atoms H)

= 0.4716 moles H

now, we'll follow with a mole to mole ratio to find
the moles ibuprofen equivalent to 2.84 x 10²³ atoms H

0.4716 moles H

(1 mole C₁₅H₁₈O₂) / (18 mole H)

= 0.026 moles C₁₅H₁₈O₂

finally, we'll convert moles C₁₅H₁₈O₂ back to atoms

0.026 moles C₁₅H₁₈O₂

(6.022 x 10²³ atoms C₁₅H₁₈O₂) / (1 mole C₁₅H₁₈O₂)

= 1.57777 x 10²² atoms C₁₅H₁₈O₂

1.58 x 10²² atoms C₁₅H₁₈O₂ contain 2.84 x 10²³ atoms H