Heres how I did this equation (if you havent already gotten the answer)
From the information you got from the question, you have 42g of CO2, and you need to find the amount of g of benzene needed to create 42g of CO2.
The first course of action to take in questions like this is to convert the CO2 into moles. You do this using the equation n = m/M (n = moles, m = grams, M = molar mass)
Mass of CO2 = 42g Molar mass of CO2 (as given to you by the question) 44.01g mol-1
Put that in the equation and you end up with 42g/44.01g mol-1 = 0.9543mol (4 sig.fig)
Now you have the mol of CO2, you need to deduce the moles from the benzene. If 2 moles of benzene is used to make 12 moles of CO2, you basically need to divide the number of moles of CO2 by 12 (to get the value of 1) and times it by 2 to get the amount of moles of benzene.
Heres the equation for that: 0.9543/12 x 2 = 0.15905mol (This is the amount of moles of benzene)
Now you have the moles for benzene, you can work out its mass using this equation: m = M x n (n = moles, m = grams, M = molar mass)
Here are the numbers in the equation: 78.1074 (mass of benzene given to you in the question) x 0.15905mol (moles of benzene worked out earlier) = 12.4g (3 sig.fig)
In conclusion, the answer is 12.4g.
I hope this helped and can be used for future reference!