find the slope of each line. set the two sides equal to each other to find b.
slope = (y2-y1)/(x2-x1)
using (2,b) and (4,6)
(x1,y1) = (2,b)
(x2,y2) = (4,6)
(6-b)/(4-2)
(6-b)/2.
that is the slope of the line with (2,b) and (4,6) as two points on the line.
find the slope of the other line, then post what you get. then we can go from there.