The data will come into your design based on a stream, so there is no need to "jump" to a specific row, it will come through the link line by line. If you want to remove a specific line you should use the operator RemoveLine. Since both inputs of RemoveLine need to be synchronous to each other you should sum the the average gray value by using RowSum. Since only the last line-pixel of RowSum -output will represent the not normalized average value - sum of all line pixels - a RemovePixel needs to be used to get the focus to the result. With an additional SYNC you can controll the RemoveLine. Both inputs of RemoveLine/Pixel need to be in sync. Independent of the text, the design below will show all details and OPs:
VA Design : LineSelectionAverage.va
Since the line length is known the sum does not need to get divided, it can be used "un-normalized".