1) What does the y-intercept mean conceptually? It's the point on the graph where the line crosses the x-axis, thus changing y-values from positive to negative or the other way around. So the point will be in the form of (0,m), thus you know x=0 at this intercept. See what to do?
2) You probably think of slope as (rise)/(run). For a vertical line, what type of fraction does this make?