Start out with ($1.50/hour)*h, which gives you the cost, in dollars, of parking for h hours:
$1.50
-------- h
hr
Next, convert dollars to half dollars:
$1.50 h 2 half dollars 3h half dollars
----------- * -------------------- = ----------------------
hr $1 1 hr
The slope you want is 3 half dollars per hour. Label the horiz. axis "hours" and the vertical axis "half dollars."
