Question 253410
r+b=42
r=3+2b

Contrary to what my esteemed colleague has said, No, your equation was not correct.
You set up 3+2x=42
Your 3+2x should be the number of red marbles.
 
42 is the total number of marbles red + black

3+2b+b=42
3+3b=42
3b=39
b=13 black marbles
42-13=29 red marbles
I don't see how you calculated the correct 29 red marbles.