what do i need to find out ? Y intercept of line l
What do i know : y = mx + c, where c is y intercept
What else do i need to know : value of m, x, and y

St:1 m=3c so the equation of line L becomes : y = 3cx + c ---> NS ; hence AD are gone
St:2 x intercept = -1/3 : 0=m(-1/3)+ c => m/3 = c => m = 3c ---> same info as given in St 1 hence not sufficient B is wrong

Now lets us combine both the statements :
0 = 3c(-1/3) + c => 0 = -c + c ; so what do you get from this ? nothing.

you have two unknowns m and c, hence you need to have two different equations to solve, which is not the case here. the information given in both the fact statements is the same. Hence (E).

Hope it makes sense... and helps.