3/5 = 9/15, so the lines with those slopes are parallel
-10/6 = -5/3, so the lines with those slopes are also parallel
3/5 and -5/3 are negative reciprocals of each other, so the lines with slopes 3/5 and 9/15 are perpendicular to the lines with slopes -5/3 and -10/6.
Yes, a rectangle is formed by these lines.