Q:

GRE geometry question: Find the angle $s$ from the figure which consists of four lines among which two are parallel to each other

I have to find $s$ from the above figure. The solution given is:
 
They say that $t$ and $3t+8$ are supplementary angles, and then proceed to find $t$ and tuse its value to solve for $s$.
Looking at the figure I am unable to see how $t$ and $3t+8$ are supplementary angles. I know that supplementary angles sum to $180$ degrees. 
Can anyone explain to me how $t$ and $3t+8$ are supplementary? Thanks
EDIT: I think I figured out the supplementary part but how did they find the topmost angle $t+8$?

A:

Because those two lines $a$ and $b$ are parallel therefore:

