there are 6 red marbles in a bag. the probabilty of picking a blue marble is 2/7. How many marbles are in the bag?

Not enough information.

Are there only reds and blues?
I will make that assumption

let the number of marbles be x
if prob of picking a blue is 2/7
then the prob of picking a red is 5/7

6/x = 5/7
5x = 42
x = a fracction ----> no way

the question is flawed, check your text.