Equation of line is y = mx + b, where m is slope and b = y-intercept.

(i) This is saying m = 3b, so the line becomes y = (3b)x + b. This is not sufficient by itself to solve for b since x and y are unknown.

(ii) x-intercept means the value of x when y is equal to 0.

Your equation becomes 0 = m(-1/3) + b, or m = 3b. Cannot solve for b by itself, so also insufficient.

Using (i + ii): They both give you the same info, m = 3b, but you cannot clearly solve for b, so it is insufficient.

Choice E.

Thank you. Should this be the general approach when dealing with this kind of questions?

Generally, yes. My approach has always been to translate what's given, along with each statement, into an equation, and then see if either of them are sufficient to answer the question. If not, I look at the 2 equations from the statements together, and in cases where there are two equations and two unknowns, you know it can be solved, however, if the 2 statements yield the same equation, as is the case here, you cannot solve it.