Ellsworth J. answered 12/09/12
LEARN FROM THE ONE WHO ACTUALLY TEACHES THE CLASS!
Use the slope formula: m = (y2 - y1)/(x2 - x1). In this case:
x1 = 4, y1 = a, x2 = 8, y2 = 3a
so plug in:
m = (3a - a)/(8 - 4) = 2a / 4
But since 2a/4 = 1/3, cross-multiply and solve the proportion. You get
6a = 4, so
a = 4/6 = 2/3