First, you should make a graph plotting "h" as the y-axis and "t" as the x-axis. Then, you can plot the equation they give you in the question.
Part A is asking about the slope of the graph. Basically, you need to find the rise over the run each second.
Part B is asking about the "h"-intercept, or the y-intercept. When Johnny first began his ride, exactly 0 seconds passed, and he was at the top of the hill.
Part C is asking you to find how long it will take to travel a third of the way down the hill. (I think the answer should be 5 seconds, but I'll leave it to you to figure out why.)