How to solve this statistical problem?

The question is:
The random variable X is normally distributed with mean 79 and variance 144.
It is known that P(79-a<= X <= 79 +b) = 0.6463. This information is shown in the image below:
http://img535.imageshack.us/img535/2671/57511082.png

Given that P(X>=79+b) = 2P(X<= 79-a)

Show that the area of the shaded region is 0.1179

Actually the answer includes ⅓(0.3537) = 0.1179
it’s just that I don’t know what the (⅓) is? or where did it come from? Can you please explain it?

Thanks in advance