y=mx+b
Value of y-intercept can be found by taking x = 0
So, y = b and hence b is the y intercept of this line.
Anything that gives us the value of b, will be sufficient to answer this question.
1. if slope m = 3 times y-intercept, it implies m=3b => b=m/3. Insufficient
2. x-intercept of line l is -1/3 => 0 = (-m/3)+b => b=m/3. Insufficient
Neither of the statements is sufficient. The correct answer is E.