Finding the slopes of sides PQ, QR, RS and SP

P(-1,5), Q(9,11), R(11,3) and S(1,-3)
using (y2-y1/x2-x1), my answers for PQ was 3/5, QR was -4/1, RS was -3/5 and SP was 4/-1
the RS and SP answers are the opposite of PQ and QR, have I made an error or is this correct?

P( -1,5), Q (9,11 ) , R ( 11, 3 ) , S ( 1 - 3 )
slope of PQ = 11-5/9+1 = 6/10= 3/5 ans
slope of QR = 3-11/11- 9 = - 8/ 2 = - 4 ans
slope of RS =-3-3/1- 11 = - 6/-10 = 3/5 ans
slope of SP = -3-5/1+1 = -8/2 = - 4 ans
so you have slope of RS wrong
PQ is parallel to RS
QR is parallel to SP