So,
Given the reaction:
The initial mass of Mg was 0.326, and, in the real experiment, we obtained 0.528 grams of MgO. Right?
Now, let's find the amount of MgO supposed to obtain according to the chemical equation:
Now, the amount that we were supposed to obtain, was 0.54g of MgO. (The result of mutiplying all the previous operations). This is the theoretical yield, what we obtained using the theory.
Now, as you discovered, the mass of MgO in the laboratory was 0.528g so that's the actual yield. This is, what you've found using the experimental process.
Finally, the percent yield can be found using the following equation:
And, we know both values, so let's just replace: