Use the formula for percentage yield.

$$\text{Percentage yield} = \frac{\text{Actual yield}}{\text{Theoretical yield}} \times 100$$

From the image, the actual yield is 33 and the theoretical yield is 42.

The first decimal place is 5, so we round up the whole number.

$$78.57142857\% \approx 79\%$$

The percentage yield is:

$$\boxed{79\%}$$