PR : RQ = 2 : 5
means
... (R-P)/(Q-R) = 2/5
... 5(R -P) = 2(Q -R)
... 5R - 5P = 2Q -2R
... 7R = 2Q +5P
... R = (2Q +5P)/7
For Q = (4, 8) and P = (-2, -2), we can find R as ...
... R = ((2(4, 8) +5(-2, -2))/7 = (8-10, 16-10)/7
... R = (-2/7, 6/7)
