Hi Chloe N.
Since 39 g of Benzene is reacted with an excess of Chlorine gas, benzene is your limiting reagent. The benzene will be consumed in the reaction to give chloro-benzene.
Your balanced reaction shows that 1 mole of Benzene reacts with 1 mole of Chlorine gas to produce 1 mole of Chloro-benzene and 1 mole of Hydrogen Chloride.
Percent Yield = Actual yield * 100/Theoretical Yield
Actual Yield = 42.0 g
Theoretical Yield = the amount expected from 100% reaction completion so we need to see what that is
39g of Benzene is
39g * 1 mole/78.11g = 0.499 basically half a mole
From your balanced reaction we know that 1 mole of benzene gives 1 mole of Chloro-benzene
So half a mole of benzene would theoretically yield half a mole of chloro-benzene if your reaction goes to 100% completion and a half mole of chloro-benzene is
0.499 mole x 112.56g/mole = 56.17 g
56.17 g is your theoretical yield
Percent yield = Actual yield*100/Theoretical yield
Percent yield = 42g*100/56.17g = 74.77%
I hope this helps.