Marcus is eating a meal that is made up of an entree that is 45 calories per ounce and a side dish that is 30 calories per ounce. he wants his meal to total 510 calories. let e represent the number of ounces of the entree that he eats, and let s represent the number of ounces of the side dish that he eats.
which function relates the number of ounces of the side dish that he eats to the number of ounces of the entree that he eats?

The correct answer is D.
 
the total calorie intake mark wants is 510.
He takes 30 for side dish (30s) and 45 for entree (45e) 

therefore 30s + 45e = 510 
now that you have an equation, judging by the choice of answers given... you have to make s the subject of formula

30s + 45 e can be factorised to 15(2s+3e) = 510

divide 510 / 15 = 34
 now we have 2s + 3e = 34

hence 2s = 34 - 3e 

didvide by 2 to make s 'free' 

 therfore you'll get s = 17-(3/2)e

Hope this helps! :)