Check the picture below.
with a rotation of 90° from the origin, we get from points (0, a) and (b,0) to the points with equivalent values of (-a, 0) and (0, b). Notice that in the graph we can map those values to a = 4 and b = 6, however, "a" and "b" are just constants and can be any value whatsoever, in this case for the sake of exemplification, they happen to be that.
now, slopes of perpendicular lines, have a product of -1, let's check these ones,
now, we could check using the a = 4 and b = 6 values as well,