Merideth's brother, miles, is 3 years younger than twice her age. the sum of their ages is 24. create an equation to determine their ages. how old are they?

Lets use O to represent Merideth's age and X to represent Miles's age, 
So what do we know? we know that miles is 3 years younger than twice merideth's age so 2(O)=X+3  and we also know that the sum of their ages is 24 O+X=24 using these equations we can input values until we find the values that work so if miles is 15 and meridith is 9 than 2(9)=15+3.   18=18
9+15=24 these values work for both equations so merideth is 9 and miles is 15

It should be a or d i’m not sure