You can notice that line B and line A can be considered as the dimensions of a figure with a quarter of the area of the total figure. It means that the length of lines A and B can be taken as one half of the length of the
Then, if the given figure has the dimensions of 5'' long x 3'' high, the length of lines A and B are:
length of A = 5''/2 = 2.5''
length of B = 3''/2 = 1.5''