So we have the following equation for the cost of sending a letter to Vienna as a function of the weight of the letter:
And we need to estimate the cost of sending a 4 ounces letter using only the graph. Since the possible weights of the letter correspond to the x axis we must first mark number 4 in said axis. The y value that corresponds to x=4 is our solution. In order to find it we must draw a vertical line that begins in x=4 and ends at the graph of the function. Then we must draw another line. This must be horizontal and must begin in the intersection between the graph and the former line. The end of this line will be a value in the y axis. This value is the quantity we are looking for. In the following graph I drew the first line in red and the second one in green:
So the y value that corresponds with x=4 is nearly y=7.50. Then the answer to this problem is y=$7.50.