3/8 of the cars are white sedans, or 37.5%
If 3/4 of the cars are sedans, and half of the 3/4 of the cars in the dealership are specifically white sedans, you can find the fraction of cars that are sedans by cars that are white sedans to find the fraction of dealership cars that are white sedans.
3/4 x 1/2 = 3/8.