The complete question is shown in the figure below. As you can see one square units is well shown in the graph. So we can conclude that the distance between two consecutive points is 1 unit. If so, then we can calculate the area of the parallelogram as follows:
9x mean 9 multipled with an unkown number and 9x is subtracted from another unknown number 'y' . you can substitute with any letters you want ,but you have to use two different letters.